I'm another guy in the "bad back" club, broke my tailbone in HS and I've been a mess since.. but to my point, I'm about the same size as you guys at 5'10" and a 30" inseam and I tend to prefer a smaller bike for my back. I use to have a 20 inch Trek (HT) and it was a fine bike, but after around 1 1/2-2 hours my back would be screaming. I picked Gary Fisher (HT) in medium which I believe is between 17-18 and it made a huge difference. I could still feel some soreness after 2 hours, but it wasn't so bad I just wanted to get off and lay in the grass lol. The fish was also a little lighter then the trek, but the size seemed to make the biggest difference.