Welcome to Chief Delphi! I think there are 2 major determining factors: cost and interest. If you can lock down 25+ students and thousands of dollars in tools and raw materials then FRC is for you. If you have the support of several teachers and 50+ students then both might be a good idea. You could do what some FRC teams do where freshman and sophomores do tabletop robotics and juniors and seniors do FRC.
To help with your decision try to talk to some local FRC/ vex teams. You can use
www.thebluealliance.com to find teams local to you.