I've used both Inventor and PTC and agree with others that AutoDesk Inventor is easiest to learn and to use. I haven't had a chance yet to download SolidWorks, but hope to do that in the off-season and try it out.
For people on your Team who are new to Inventor, please feel free to download our free, 5-session introduction to Inventor (geared more towards Robotics designs) that our Team (#2530) put together:
http://frcteam2530.org/resources