Dan I think it's true the CC has a bit more rear legroom, maybe an inch or two. The QC cab structure is different and more beefed up for strength, the rear windows do not interchange. Also if you park a QC next to a CC you'll see the cab is almost an inch wider (the sides of the bed are basically flush with the back of a CC cab whereas the QC cab is flared out a little). I think with everything being the same a QC truck weighs about 100-150lbs more as well.
The Quad Cab came available pretty early in the '98 model year, maybe even at the beginning. . . I've seen plenty of 12-valve Quad Cabs.
After a few rounds of going off-road in my '01 QC it didn't take long for the cab to feel loosey goosey. I also have a '96 CC which at 429,000 feels stronger and more solid than my '01 did at 40k miles.
The other cool thing unique with the '98 and '99 club cabs is the passenger seat slides all the way up to the dash, can't do that with a '95-'97 CC. It's much easier getting in the back of my '98 than the '96. Interesting Dodge went through the trouble of making the special seat slider that ended up going in a limited number of trucks over 1. 5 model years.