Bigger is ALWAYS better. If you end up doing a lot in the 300 and get popular, then you know you'll want a larger smoker.
Guess it depends on what you want to do it with. You don't have to go larger if you're never going to do large amounts.
Do you want more mobility or will it be stationary? Say ribs, if you don't need the larger capacity, the 300 is certainly easier to maneuver.