My advice is, get the right bike for what you need it for. If you're going to be doing off road riding(I'm not familiar with the trails in Santa Cruz), and that's the main reason for getting the bike, then get a mountain bike. If you're thinking mostly about commuting on paved roads, I suggest you get a hybrid. Although many people commute on mtb and love it.
Go to a bike store, have somebody help you choose what you need and figure out your size. I wouldn't recommend buying a bike on line in your case, when you're not really sure what you want. Test ride a few different bikes and see how they feel.