I once had only two bikes: Both road bikes: One with 170 and one with 175 cranks. I noticed that it felt more smoothly professional to pedal the 170s after the switch from the 175s. When I switched back to the 175s, it felt slow. Slow is hard to do smoothly or professionally but in my head, I'm thinking about having more "leverage" which is good (right?). Then, we had a guest instructor at a club clinic one day who was my size and he prefers 172.5s. So that is my go-to these days. I'm still slow, but smooth, and not professional.