Launchers are more expensive, harder to aim, and take up twice the space as three Rockets of the same payload. Primarily, this is because Payload Size can only be half of the Launcher's size. Firing a Size 2 Explosive requires a Size 4 Launcher, which will run you 14 CP (12 for the launcher, 2 for the Explosive) and a Use of 8. A Size 2 Rocket, on the other hand, costs only 4 CP with a Use 4. The cost doesn't balance out until you carry six rounds, but you could only fire that many in a battle much longer than normal. Multiple Rockets can also be fired at once, while Launchers can only fire once a turn at most.
It seems to me that Launchers should, for a start, be much cheaper. Without Explosive payloads, they're a harder-to-aim, longer range, half-damage Gun for the same cost. Also, to make them more valuable for long-range artillery, they should either get a range increase or the range of Rockets should be decreased (I'm leaning toward the latter).