having done both.... Calif, Channel Islands in particular, wins hands down!
Kelp forests are unique and in warm water locations you don't see incredible, colorful fish like those that live there! California you can see nudibranchs of all colors and types, as well as sharks and sea lions, so the variety of sealife, big and small, is better.

We have done Kona Hawaii, and although the diving was enjoyable, all the reefs started looking exactly the same by the end of the week, and not very colorful. Fish were cool but ....
We are heading to NC coastline to dive there in July... so my opinions may change.
robin