San Diego = Nice climate for biking.

The distance is probably too far to go on a cruiser/comfort bike, at least on a regular basis. I mean you certainly could but I wouldn't recommend it.

Don't get me wrong, it's not a terribly long commute, but it's not a short one either. If your commute was really flat, a single speed might work well and not be too hard on the wallet. Something like a Giant Via. It's a little more upright than you might want for that distance though. It's far enough that I'd be thinking of a road bike but new ones are hard to come by in that price range.