It's been a few years since I was at the Casa Magna Cancun. Nice hotel; got an upgrade as a Plat. It was definitely much cheaper than Grand Caymans.

If you're going to scuba dive, Grand Caymans is much better.
If you want to sightsee a bit, there's more to see outside of Cancun (Tulum, Chichen Itza). We rented a car for cheap and drove to both. My wife and I felt comfortable travelling all around Cancun and the surrounding tourist sites - this was during the height of the 'druglords are executing everyone' scare.

If we were to go on vacation tomorrow and had those two choices, we'd go to Casa Magna but YMMV. Your tourist dollar will go much further in Cancun than Grand Caymans.