why are you going for them?
depending on the price, it may be more advantageous to go with a track crank over a road double. mainly for chainline compatibility, but additionally the .6 cm shorter crank buys you a few degrees of pedal-strike-free bank into turns which could be pretty valuable depending on the bb height of the frame you're building up.*

that said, the optimal chain ratio depends greatly on 1) whether he'll be doing stop-and-go riding (in a city for instance) versus long uninterrupted rides, 2) whether you live in a flat or hilly area, and 3) what kind of rider he is (high cadence vs masher).


*pedal strike isn't nearly as important an issue if you're using a freewheel, btw.