Depends on the technology used, how big is the fleet, what are distances of travel (inner solar system, Jupiter, outer gas giants, interstellar), methods of travel as o.m. noticed, velocities, desired ISP(and thus the efficiency of reactive mass usage)
Mars-Earth-Venus-Mercury - Solar-powered crafts make total sense.
Jupiter-Saturn-Uranus-Neptune - any kind of fission-fusion makes sense.
Interstellar - fusion, Bussard ramjet, beam-powered propulsion if one likes fast travel, but solar-powered, etc also fine if time is not a problem but energy efficiency is(bulk transporting).
Mass drivers will be a good option for the whole Solar system, for traveling between planets, space rings, and space lifts are a good option for transporting to/from gravity wells.
Thus, until we invent something superior which can easily replace all those methods, and light robust fusion reactors are candidates for that, all those varieties may be used.
We struggle not to predict what can or may be used, but to predict the demands for particular needs as a function of time.
Amount of energy from fission fuel isn't bad actually - it is about 0.8MeV per nucleon(235U = 200MeV), for fusion it is about 3.8MeV per nucleon the most(2D + 3He = 18.3MeV).
0.01m/sec2 - for solar system it is not bad as an acceleration speed, 864 m/sec per day, or 17 days to gain 15 km/sec. 15km/s is enough to travel across the system, not fast but still. The same speed is not too much for mass drivers too(to launch a craft) and as result, we may expect a combination of approaches as mass drivers can be used to launch a craft, and engines may be used for correction and slow down at the target orbit.
But is that combination viable depends on such things as, for example, travel between planets may be significantly faster and cheaper than travel from a planet to an asteroid because mass drivers may be used and for launch and for slow down (this way we also would be able to transfer the energy between bases)
As asteroids and small bases may have troubles(and definitely will have troubles) accepting high-velocity ships.
Mass driver launching needs infrastructure across the Solar system, reactive propulsion does not need such infrastructure as it relies entirely on itself. However mass driver launch is way more energy efficient, mass efficient for the ship, allow to transfer of energy between bases which may be important for remote bases(let's say at Jupiter) if no fusion is available. And because of those mass drivers, it alone may allow establishing bases on all significant bodies in the solar system, even without fusion.
So, what comes first orbital ring or a good fusion reactor-engine?
We just do not know, both allow us to solve the problems(travel, energy) in a slightly different fashion, this way it just depends ... on people, on their interests.
Fusion will be used for sure, and it is superior, just because there is a lot of fuel for it in the universe, in the solar system, easy to get, easy to use(with technology for doing that).
But for a small fleet - 100-1000-10000-100000 ships in the solar system - fission is good enough. To deliver 1 cubic kilometer of liquid hydrogen, from Jupiter, it needs to spend 800 tons of fission fuel and 10% of that hydrogen as reactive mass. And this cubic km of hydrogen may be converted into 567 million tonnes of water (-10% already), and to produce 819 million tonnes of aluminum, or 1732.5 million tonnes of Iron as a byproduct at almost 0 energy expenses - in fact, it can be used to reduce all matter to its elemental form from a 950m diameter space rock(aka asteroid from inner parts of the solar system with average density 3000kg/m3 of typical rock, a solid chunk, which they aren't, mostly)
All that is just at expenses of 800 tons of fission fuel, but we mine a bit more than that, 20 times more(at least) than that, and we do not get that many exciting results for our efforts to mine it, I mean energy-wise. I would say spending 800 tons for the result is totally worth the spending.
Back to OP premise, a side note about antimatter production
A note about antimatter and energy of its production, if the technology exists, it can be used the same way as fission can be used - basically, it is the ultimate type of energy accumulation, spaceship rechargeable battery. Even if total efficiency is 0.01% (production, storage, use, conversion to propulsion) it can be used. 1000x1000 km mirror foil at earth orbit may help to produce 4765.44 kg (at 0.01% efficiency of production) of antimatter per year. It is not a big energy production facility(1000x1000km is not a lot in space), and it is pretty a lot of antimatter because 1 kg of antimatter is equivalent to about 1 metric ton of fission fuel (energy of fission fuel 235U is about 0.00091mc2), this way same 800 metric ton for fission fuel is equivalent to 800kg of antimatter and such station may produce fuel to transfer 6 cubic km of hydrogen per year, which equivalent to refining of a lot of matter, and possibly those materials are enough to build 5-20 new stations. Therefore despite difficulties of antimatter production, it may outweigh the fission and maybe even fusion since you do not need to search and extract those fuels, but instead, you can create antimatter from nothing using the energy of the sun, which at the moment is just dumped into the space void, and which is plenty of available for cheap.
The 23rd-24th century - we may or may not use all Sun energy for other purposes, therefore, production of antimatter maybe not be a problem energy-wise, despite the efficiency of the process, and may be capable to cover all our needs in antimatter at the time. But if you choose it, it may be a good idea to place production farther away to increase the efficiency of the production, Gas giant shadows, or near Neptune(or planet X maybe(?)) and beam energy needed for the production from the inner solar system, because at farther distances you can recuperate energy you lose during the production and increase the efficiency of the process.
At Neptune orbit, solar irradiation is about 1.5W/m2 it is equivalent to 71 K temperature(black body) and if you lose 99.99% of energy during the production of antimatter, and waste heat is at 1000 K, you can recuperate the 92.9% of waste heat during each cycle and overall efficiency of the process of this imaginary antimatter production facility will be 0.14% instead of 0.01%. And that is worth of travel actually, worth of a "trade" ship.
Back to OP problem
As you may see already, the problem is not what we already know, because indeed our current knowledge probably allows us to look at a bit distant future than we could do 100 years ago, and they were damn good at it (Nikola Tesla, Jules Verne - we are just not enough in the future to fulfill some of their ideas. As I have discovered recently, Nikola Tesla proposed Orbital Ring more than 100 years ago, Jules Verne with his indestructible(at earth depths) fast-moving submarine just a prophet(I know how to build it:) ) and cannon mass driver(also good enough for looking in the future of maglev mass drivers))
But the topic is broad, for different situations for different demands and activities there are their own best solutions.
Fission is good enough for star systems.
Fusion is ok almost everywhere even with interstellar, but it does not shine with interstellar, except Bussard ramjet, but very good for a star system.
Bussard ramjet is relatively good for interstellar travel but inside star systems are not good at all.
Antimatter is ok for a star system, and ok for interstellar if the efficiency of production is good enough, but it does not shine with interstellar travel.
Beam propulsion is ok for a star system and good for interstellar travel - if combined with some of reactive propulsion or infrastructures in destination point, interstellar variant need a bit more advanced infrastructure (I keep forcing that technology even if no one seems to like it, it is ok to be used at destination point to catch those beam(or mass driver in this case) accelerated crafts, some use case applied to disassemble planets is here)
I would recommend sticking with mass drivers, they are always handy to lift things from gravity wells and to be used upon all massive bodies - in form of tracks, launch loops, orbital rings, not only to launch but also to give a surplus delta-v kick for those crafts, consider it as an advanced version of gravity slingshot maneuver.
As the energy production for reactive propulsion to fission, solar-powered(not necessary solar cells), fusion.
As for reactive mass to Hydrogen from Jupiter.
I would avoid solar sails as not an energy-efficient solution, as for travel in the star system, but an acceptable solution for interstellar travel as it is more mass efficient than other solutions with reactive propulsion, but it is demanding for infrastructure in both cases.
I would avoid using Project Orion as a good example for a spaceship because it is a very inefficient design. It is not good at multiple levels. I salute all those people who spend their time on the project, as it is very useful in moving us forward in understanding what is good and what isn't so great, and at that time of the project, it was the great spirit which I would like to see today and applied to our current technologies. Basically, it was great at that time, but its intent wasn't to foresee the future but apply known technologies to the problem of interstellar propulsion at that time.
Also, I recommend defining which kind of "trading" will it be in your setting, because everything isn't so easy and you may take look at some space-based economy questions [economy][space-colonization] and [economy][space]