Any realistic antimatter weapon would resemble a traditional light bulb more than a kinetic energy impactor, and considering the mechanism of energy release, accelerating antimatter to high velocities really does not add anything in terms of damage (although for targeting solutions and to ensure the antimatter makes it through any defensive screen, a high velocity delivery system is a must).
The antimatter must be held in a vacuum to prevent it from interacting with matter before it is supposed to. It should also be cooled to as close to absolute zero as possible to make it relatively non reactive (no stray anti particles wandering from the surface of the antimatter bundle), and it would have to be suspended within a powerful magnetic or electrostatic field to keep it entered in the container.
Now since a microgram of antimatter reacting with a microgram of matter is the equivalent of 43kg of TNT (thanks to the atomic rockets boom table), a device carrying a pound of antimatter (@ 500 grams) should have the energy release of about 21.5 megatons. To give you an idea, modern strategic nuclear weapons are thought to have a yield of about 300 Kilotons. The only semi plausible suggestion that a 20 megaton weapon was ever deployed was speculation I read once that a regiment of SS-20 "Satan" ICBMs was held by the former USSR to carry one 20 megaton weapon each, with the sole purpose of turning Cheyenne Mountain into Cheyenne Lake.
So the question is what would EarthForce be targeting which requires such firepower? Have the Martians dug into Olympus Mons or the other Tharsis volcanos? Is Earthforce going to crack open the moons of Mars and neutralize them as potential orbital installations? Are the Martians hiding under the polar icecaps? Since the creation and use of antimatter is going to be very expensive and potentially quite dangerous for the user to handle, these targets must be extremely high value. Using antimatter for ground bursts provides some of the energy release will be converted into seismic activity, and the shockwaves travelling through the ground could collapse tunnels and bunkers, but airbursts could potentially sleet wide areas of Mars with highly energetic gamma radiation, as well as blasts of exotic particles moving at close to light speed, in addition to massive blast waves travelling through the atmosphere. Any surface installations would almost certainly be destroyed (like solar panels and communications antenna).
Of course there is a simpler way to get that sort of bang for the buck and more:
There was a three page paper: Nuclear explosive propelled Interceptor for deflecting objects on collision course with Earth. Johndale Solem, Los Alamos, proposed unmanned vehicle. No shock absorber or shielding. The pulse units were 25kg bombs of 2.5 kiloton yield.
Get to high velocities with only a few explosives and small shock absorbers or no shocks at all. Launch against a 100 meter chondritic asteroid coming at 25 km/sec. 1000 megatons if it hits. Launch when it is 15 million kilometers away and try to cause 10000km deflection. A minimal Orion weighing 3.3 tons with no warhead would do the job. 115 charges with a total of 288 kiloton yield. Launch to intercept in 5 hours. Ample time to launch a second if the first failed.
So this beastly device can be launched directly from Earth or the Moon, accelerate at 100g and strike with a gigaton of energy on impact. In terms of cost, I would suspect that since most of the parts would be churned out on an assembly line, and you have no exotic antimatter containment to deal with, it would be much cheaper to have these in squadrons based on the Moon or maybe even on NEO's, to provide the most dispersion and coverage for EarthForce's premier strike weapon.
And don't try to run away:
Another aspect of the fast acceleration that is possible is that an unmanned Orion go from earth or earth orbit to Mars (decelerate at halfway) and get to Mars in under one day going at 100Gs if Mars and Earth are in the close approach. If the unmanned version was going at 1000Gs (which was a design that is possible), then Earth to Mars could be done in a few hours. At about 300Gs and you would be looking at a Mars Overnight package delivery.
Of course it is always possible the Martians have been thinking along these lines as well.....