I've got 172.5 cranks on my Sputnik and I'm glad. It gives me some extra torque to push over the hills and against the wind. At 64 I need everything I can get. I, too, have about a 32" inseam. I would go for the 170. It used to be standard on multi-speed bikes - not sure if that's still the case with 20+ speeds.

Shorter cranks will let you spin faster but longer ones let you come off the line quicker.