[marypim]

Personally, the forklift seems like the best idea to me

Pneumatics, Springs
Pros: Faster, and they may fit in the minimum size dimensions (no need to unfold)
Cons: You'll need a LOT of energy to launch a 10 pound ball 6 feet in the air. The springs to do this would have to be huge, and I doubt FIRST would allow it. Also, how would you reload the springs for more than one hurdle?


Catapult
Pros: I can't really think of any...
Cons: It would have to be huge to get enough leverage to launch a ball, so it would involve some serious unfolding to get out of the minimum size box. Also, when you do have it unfolded, moving would be problematic.

Forklift
Pros: There aren't very serious cons xD
Cons If it becomes top-heavy, your robot could tip. Pay attention to balance.