650 c wheels- what is the tradeoff?

[Eden]

If you race see eden's comment. also at UCI sanctioned race, 650C wheels are not allowed for TT's (atleast that is what I remember).

As long as *both* wheels are 650 it's OK (at least it is for USACycling - better be cause my TT bike has 650's - and I don't think the rules vary too much from UCI). You can't have an odd wheeled bike - think getting ultra aero, by having a smaller wheel on the front than on the back.