If you test ride the bikes and they all feel the same to you, it doesn't matter much. I don't think anyone really knows what the difference it between all the various levels of carbon that bike manufacturers use. The higher model frames are more carbon less filler. That makes them lighter. As for other claims of stiffer, more vertically compliant, blah blah blah, I don't know about that. If you have back issues, you might want a bike that offers the smoothest ride. The reviews I've seen in these forums seem to indicate that the Roubaix has a great ride. The Giant Defy isn't as popular a bike, but I have seen reviews that say the Defy has a harsh ride.

The Roubaix is a very popular bike and the Zertz inserts give it a smoother ride. If you have back issues, the Roubaix seems like it's the best choice despite the higher price tag.