Since you already have the bike, I'm not sure if my information will be any help, but I'll give it anyway. I have a 31" inseam. For normal riding on a road bike 170 or 172.5 work fine for me and I don't really know if I can tell a difference between them. I have one road bike and a cyclocross bike (which I mostly ride on a flat rail-to-trail) which each have 175mm cranks. That is uncomfortably long for flat land and spinning - I can definitely feel the difference, but it works OK onbig hills. My full touring bike and mountain bike both have 175's and feel fine, but they are each used in lower-rpm/high torque situations and I like the extra leverage.

From your comments above, if you could definitely tell the 170's were too small I'd expect the 175 to be about right. The slightly lower pedal cadence also sounds probable.