I have never owned a Look but a friend of mine used to race with the 595, and he was very happy with it. I tried his and liked it - fast, responsive, and lightweight. Very good bike.
If you have back problem you may want to try the taller setup, I think the model with the more relaxed geometry is the 585 but I could be wrong, just check on their catalog, I know they have one that's a more relaxed ride. Good luck and congrats to both you and your hubby for new bikes