I can't imagine the ride quality would differ in a way that would be significant enuf to be noticeable for ya. The CAAD 7 would be a bit lighter, but I would hazard that the CAAD 7 just won't be as durable for the long term. I think they even have some sort of advisory that they are high performance and not intended for years and years of hard service. If you ride hard and ride alot the CAAD 5 would be more of a lifetime bike and less expensive too boot. Personally, I intentionally opted for a CAAD4 R4000 frame when the CAAD6 was the top offering. I intentionally wanted the aluminum expertise, stiffness and tube shaping of Cannondale without an integrated headset and without the proprietary Hollowgram bottombracket, I just think these are questionable "improvements" to an already near perfect product. I like traditional headsets and wanted to be able to use readily available components in these areas.
If you're talking aluminum, C'dale has been doing it longer and better than anybody, but I really think C'dale reached it's apex in the CAAD4. It had the wonderful tube shapes, hourglass seatstays, lightweight, 7005 aluminum, stiff, fast, rugged but tolerably comfortable. The Lion King won 4 stages of the (99?) Tour on his CAAD4. IMO the CAAD 4 was their "high water mark" as a company.
After the CAAD4 they started integrating the headset with the CAAD5 and then the special bottom bracket on the CAAD6 and then to the different aluminum alloy (Optimo?) for the CAAD7. The financial and bankruptcy woes also came along as they counted 5, 6 & 7. Did quality suffer? Did customer service?
Anyway, this long winded post is my way of saying that I'd get the CAAD5.
It's less expensive right? The weight savings is negligible, and I believe it will provide more reliable service for more years. You can also use a conventional bottom bracket rather than C'dale's proprietary one, if I'm not mistaken.
By the way, I love my 'Dale! Despite all the criticism they receive, they make (made? no, MAKE) a good product. My bike is SMOKIN' FAST! and it's beautiful with a fire road fade paintjob and Kysrium SSC wheels. I marvel at how it looks sometimes, it's like a friggin' rally car or something.
Get the CAAD5, you really don't want the "ultralight" (stupid light) aspect of the CAAD 7 unless you're a racer or plan to buy a new bike regularly.