[gburlison]

I voted, mentor designed/student built. I believe it is the best of two bad choices. As many have said, it is hard to design something that you have never built. As an electrical engineer, I have little practical experience in mechanical design. I remember the first thing that I designed in inventer. I spent a lot of time chamfering edges, filleting angles, in general making it look good. It was impossible to machine with the tools we had available.
By learning how things are made, a designer is better able to create workable designs in the future. I think that the best system would be for new team members (students and adults) to learn how to build the robot, and veteran team members (students and adults) can design the robot. This provides a progression from the novice to the expert and helps students learn how to be mentors, since the veteran students can be mentors to the new members as they learn how to build the robot.