That isn't really true if you compare apples to apples. Sure, if you just look at the price of a hotel package and diving, Coz is much cheaper. But if you compare the typical "cheap" Coz package to someplace like Little Cayman Beach Resort's all-inclusive package, you have to include the cost of taxis in Coz, or the time wasted walking to restaurants, the cost of being disturbed by traffic noise, the value of not being annoyed by street vendors, the time to wait while your dive boat visits multiple docks picking up divers, the dive time lost when your group has to surface early because half the group is composed of air hogs, etc.
I like Coz, and I like the Cayman Sister Islands (haven't been to GC). But I like them for different things. If I just want to do a quick cheap dive trip, Coz is great, convenient, cheap, not particularly relaxing per se, but you can get your dive fix. But if I want a "vacation", then the Sister Islands are the only way to go...no crowds, no taxis, no noisy traffic, no getting harassed by street vendors, etc. Just quiet beaches, valet diving, terrific food, and nothing to do but eat, sleep, dive, and relax.
Kdufour,
To explain a little more what everybody is talking about with drift vs moored diving...as you are familiar with in Cozumel, when you drift dive, you all enter the water together, you all stay together on the dive with a guide, and you all surface more or less together, usually when most of the group is short on air. When you dive from a mooring as most dives in Cayman are done, you enter when you are ready, you and your buddy alone pick where you want to go, you pick how fast you go and how long you stop to look at anything, and you pick when you come back. Obviosuly there are some limitations, but if you are like me and hate the "following the crowd" feeling of drift diving, then you will LOVE diving in the Caymans. The best diving really is on Little Cayman or Cayman Brac, but the East End of GC is reportedly very close. If your family really needs more topside entertainment, then you are probably better on GC; but if you are all about the best diving and don't need nightlife, then the Sister Islands are where you want to be.