It's amazing how you can really feel a few mm difference. I'm 5'6", 31.5" inseam. I have a bike with 170s and a bike with 172.5s, and I can tell the difference. I prefer the 172.5's because I get better leverage -- it's noticeable all of the time but particularly on hills. I suppose it's a tad easier to spin faster on the shorter cranks, but the 172.5's allow me to spin as fast as I need to (normal cadence on 172.5's is around 90-100; when I get really tired at the end of a windy ride, I might ride in lower gears and spin between 100-110).

When bike shopping I often feel lucky (perhaps even grateful) that I'm very average sized (that is, I'm not petite). I am on the small end of the range where it's reasonably easy to find components to fit (and it drives me nuts how few models of bars come in 38cm, and that my size XS, WSD bike came stock with 40s), but I think it's still much easier for me than for many smaller women.