I think the answer would depend (as noted previously) where you would plan to live on the Big Island or on Maui.
Have you considered other factors regarding day to day life in which island to choose - price (and availability) of housing, traffic congestion, driving distance to work, etc.?
Nothing wrong with basing your decision solely on which island has better diving, if that's what you want to do - but the two islands are pretty different in other aspects which may or may not directly impact how you feel about living there.