Either place is FAR superior to Roatan (where I've been twice, but never plan to go again), as far as I'm concerned. Roatan's primary attraction is that it's CHEAP, but its wrecks and walls aren't as good as those in the Caymans, and its fish life (what little there still is) is far less than either the Caymans or T&C's.
As Giles says, it depends a little on which islands, but in general I agree with his suggestion of doing Grand Cayman for awhile and Little Cayman (or Brac) for a few days, too. You might even consider the Cayman Aggressor - I've been on it twice, and they've got a good routine, although I'm not sure if they're going to be making the crossing over to Little Cayman when you'll be there.
T&C has some surprisingly good diving, especially if you're going to be doing a liveaboard, but the Caymans are better IMHO (and I don't live there, so my opinion is more objective!
).
One other consideration is that Grand Cayman is an excellent place to practice with your new DSLR - little or no current on most dives, with great vis and a few really unusual dives (e.g., Stingray City, Tarpon Alley). It is where I always recommend that new divers or new uw photographers go to get some beautiful, easy diving experience under their belts.