Depends on the type of riding you do bu there are a lot of good places. Nor Cal, close to the sierras is good. So Cal is good too, just have to drive to get to all the spots and its a little more crowded.
wooo go San diego got all different types of freeride and DH. got some northshore stuff if you know where to go and got lots of fast DH trails. unfortunatly they are spread out around the county...
well if you are deciding on school then don't make biking a factor. but, I would say nor cal is better because i live there and there are all sorts of places that are completely legal to ride and build. You have Santa Cruz which is awesome and everything else within driving distance.
I'm moving there to live and work (if I have to) for like 2 months because I dont feel like going through a full winter up here, all I want to do is bike, hahaha