When it comes to an airship — and since you've said there's magic involved but didn't explain how, I'm going to ignore it — the problems are (a) lift and (b) being pushed around. Note that I can't imagine this to be a comprehensive list.
Wind For example, the Westerlies. There are "wind currents" just as their are "sea currents." Your trade routes will want to avoid head winds and take advantage of tail winds.
Mountains Altitude is normally an issue with airships. The higher you must go, the more costly and dangerous it is. You'll want to avoid high mountain peaks and tall ranges, preferring to slide along valley chains and take advantage of low passes. Also, generally speaking, note that wind gets worse with altitude.
The Sun As the earth rotates, the sun "pushes" air as it heats it (thereby expanding it). This causes more wind that can be predictably used or avoided. Think of canyon winds, which tend to be nasty in the morning and evening. Canyons "breathe" (air rushing in and out) and how they do depends on whether they're low point faces east or west. (BTW, this action is part of what creates the Westerlies).
Wind Shear Wind shear is a very strong burst of wind, usually downward, and usually caused by meterological complexity, such as occurs around storm fronts or near mountain ranges. Denver International Airport is famous for the problems of wind shear. Without power, these need to be avoided and trade centers will (generally) develop in areas of low wind shear.
Thunderstorms & Tornadoes While thunderstorms can occur almost anywhere (and tornadoes less so, but still), there are areas of our planet where they are very common. Trade centers will want to avoid these areas. Note that both thunderstorms and tornadoes (and hurricanes) tend to create rotating wind, with will want to force an airship to turn, or may even cause it to spin like a top.
Lift Powered flight can deal with, for example, the poor lift of very hot areas. Air expanded from heat is less dense and therefore provides less lift. This makes it very hard to, for example, take off with a fixed-wing plane.
Airships react to hot and cold in the opposite way to fixed-wing flight, but they will nevertheless have similar problems depending on the nature of what gives them lift. Ignoring magic, if they're filled with gas, that gas will condense in cold — making polar transits difficult due to lower lift and the need to additional gas to compensate — and expand in hot — making equatorial or desert transits preferrable due to needing less gas to produce the same lift.
Water vs Land Routes For this issue I don't have enough background. My guess is that land routes will have more predictable wind and weather patterns and thus be more desirable. But, honestly, I could be wrong.
EDIT: One last thought. Sea routes are intrinsically limited in that you must have a port on the sea. Sea ports are desirable because the port is protected (a good harbor) and deep enough to handle the boats. They're also located, ideally, where train/vehicle access can easily get goods to the port, but that's secondary to the other considerations.
The considerations for air routes are different. Obviously, a port located within a circle of high mountain ranges is a problem, but why would you do that, anyway? You're economic trade centers would naturally form at area where the airships are swiftest and least costly to operate. Along Westerlies, a low altitudes and away from mountains. Etc. Sea routes almost must be developed second to the ports due to the absolute needs of ports. Airships are fairly the opposite where they might develop second to take advantage of the best routes, which won't be "port locked."
