SolidWorks is also an option, but you must apply for the free sponsorship for your school/organization. It takes a few days, then you will generally receive enough licenses for your team.
Solidworks part libraries are available through “3d content central,” and by the efforts of a couple teams. Also, .step files, available on the vex robotics part pages are universal format files that can be used by any 3d CAD software.
You might consider checking your local Community College program to see what software is being used by the industry in your area. In the greater Detroit area, few people in industry use Inventor, and even fewer use Fusion.