If you make the decision purely on diving, Grand Cayman wins hands down. So much variety of life and of types of dives. That being said, Maui is a wonderful place to dive. I have had great fun with Ed Robinson's (dive charters) and experienced a couple of "Oh, my God, I can die now" moments. Doing a night dive while listening to whales sing is a truly amazing experience.
As for Grand Cayman, I did my first 100' dive there with no problems or worries (or chills-they have no thermoclines.) I also did my first night dive there with no trepidation.
Either way, you win.
K.