I'm currently on a 49cm Lemond Buenos Aires. This is a men's frame since my torso is longer, and I feel like I am falling off of the front of Lemond's WSD bikes. My bike is the steel/carbon spine model; however, if I were purchasing this year I would definitely go with their Triumphe carbon. I rode those last August at Trek World and absolutely loved the ride.

Since you have some back issues, I would stick with carbon, steel or titanium. Aluminum would probably be way too uncomfortable.

Good luck - have fun test riding.