I switch between bikes with 170, 172.5, and 175mm cranks. I can't even tell difference apart from the over all feeling of a completely different style of bike. 5mm is nothing, it's only 1/5 of an inch. If you're tall get 175mm just because, if you're short get 170mm just because, If you can't decide, get 172.5mm, but there's really not much difference in my opinion.