I've been to Grand Cayman and Curacao. Of the two, I'd rate Cayman higher on the tourist scale.
The diving on Cayman tends to be deeper, clearer and there's more big stuff per dive. Curacao is like Bonaire (same reef) in that there's a lot of shore diving and smaller stuff to be seen. In a week on Cayman we saw Turtles a couple of times, Tarpon, Rays and lots of fish. Some sharks in the distance out on the wall. On Curacao we saw turtles, octopii, barracuda, moray and other eels and lots of different fish. Even a seahorse. I also did the Dolphin Dive at the Sea Aquarium - $200 buys 1/2 hr. with a pair of dolphins on the reef.
Topside Cayman is pretty quiet except when the cruisers are in port downtown. (daily) Curacao you don't notice it as much since they tend to stay downtown. Either place is really crowded until the ships leave. Willemsted still stays open for the locals/tourists, Georgetown shuts down after the last boat leaves. Mostly because Willemsted is a much bigger city.
The West End of Curacao is pretty isolated and spread out between dive locations. Most of the good diving off Cayman is by boat, on Curacao you mostly go to the exceptional sites via boat.
Curacao is much bigger, more rundown and a lot more industrialized. It's a deep water port and there's a refinery just north of downtown. Probably several times the size of Cayman also with a lot more people. Some of it is kind of grimy also, hookers and such in one downtown area. Cayman wouldn't allow that. Curacao has about 15 casinos, Cayman doesn't have any.
Of the two, I'd say Cayman was much safer. Since it's the offshore banking capital of the Caribbean. On Curacao every resort had security and one of the places we stayed had industrial strength bars on all the doors/windows. We were warned not to leave anything valuable in the car at isolated dive sites. On Cayman we left extra gear sitting on the dock without incident. fwiw, we didn't have any problems on Curacao either but there are reports here of thefts that have occurred.
Cayman was more expensive in 04 than Curacao was last April. The dollar is something like 20% negative to the CI$ so everything costs more. Don't know if you've been to the Bahamas but Cayman was pricier than Nassau.
What are your property choices? On Cayman, one of the big timeshares is Morritt's on the East End, it's a 45min drive back to civilization from there.
Personally between the two I'd go to Cayman. It's much nicer topside and the diving is a wash in either place, both are really good just different. Except that a lot of Curacao tends to look the same after a few dives.