My opinion is that FRC virtually requires adult technical mentors while FTC teams benefit from them but does not require them. FTC teams can get started by simply building using the kit parts, but the best FTC teams will often have 3D printed parts, CNCed parts, laser cut parts, and/or machined parts on their robots. A few have even used 3D printed parts as mold models and used a foundry at their school to mold custom aluminum parts. About the only thing I haven't seen much of in FTC is welding.
I know some kids who are inspired by working with adult mentors on FRC teams and some kids who strongly prefer being able to "own the process" more on their FTC teams. Different kids are inspired by different environments. I think it's great that FIRST offers both.
If you're interested in this question, you should check out
this report from 2011 commissioned by FIRST. In particular, from page 7 of the report:
"FRC members were more likely to
report increases in their interest in science and technology (97% vs. 95%), in their plans
to take science or math courses (90% vs. 86%) and in their interest in going to college (92%
vs. 87%). FRC team members were also more likely to report learning about key values,
including Gracious Professionalism (96% vs. 90%) and volunteering in the community (83%
vs. 74%). Team leaders reported a similar set of gains. FTC team members, on the other
hand, were substantially more likely to report an increased interest in computer
programming (91% vs. 78%) and were as likely as FRC participants to report that they were
interested in science and engineering careers (85% vs. 83%). FTC and FRC team
members were also equally likely to report gains on questions related to life and workplace
skills (FRC members were more likely to report gains in communications and cooperation
skills, but there were no significant differences between program on the responses to the
other skill questions). "
The numbers are pretty darn similar, showing the very similar benefits of both programs, IMHO.