I don't own a Look but I *thought* they were mostly a racy-aggressive type of build. It may not be the best bet for a bad back, can you test ride one? The Orbea Diva is a pretty aggressive style bike, it is supposed to know one speed for riding - fast. At least that is what the women I know locally that ride it say about it.
If not are you comfortable comparing geometry charts? I did that with my Fisher because I couldn't ride one but I knew what I hated about Specialized and my old bike's build so I was pretty confident looking at the difference in the Fisher and knowing it would be better for me.
The Ruby is a slightly more relaxed build, the aggressive Specialized for Women is the Amira. I think Madones are a bit more aggressive but I know a lot of people who ride them on Centuries and long rides. The Cannondale Synapse is carbon and geared as fast (stiff) but comfortable bike. It is more relaxed than their race line bikes. Can you rent the bikes you are thinking about? My LBS rents Madones, don't think they do the Ruby but a parking lot ride won't really let your back know if it likes the bike.
Carbon will be way better than aluminum, I suppose. I went from steel to carbon and noticed no improvement with carbon for comfort. I still get road vibration on carbon since we have a lot of chip seal in Texas, so nothing will kill it all. Carbon is stiff and light though.
Amanda
2011 Specialized Epic Comp 29er | Specialized Phenom | "Marie Laveau"
2007 Cannondale Synapse Carbon Road | Selle Italia Lady Gel Flow | "Miranda"
You don't have to be great to get started, but you do have to get started to be great. -Lee J. Colan