Both. Neither. Ok, here's the thing: classical wisdom says keep that thumb behind. School of rock says get that thumb bent around. Just like you, I was confused about this. Best rationale I've heard so far is thumb behind when you need to stretch and reach notes on the fretboard. When you need to bend, wrap the thumb around to give your fingers support.

But what about in between you ask? Ok, the truth is, most rock players play with their thumb wrapped almost all the time, slipping it behind the neck only when they need to. If you're shredding and really reaching a lot, you may tend to keep the thumb behind more, but most rockers will keep that thumb wrapped. If you want to confirm it, just watch some great players out there on YouTube and you'll see that they wrap their thumb almost all the time.

A word of advice: do what feels comfortable, and be VERY careful about putting a lot of pressure back against your thumb, especially when its behind the neck. If your thumb is behind the neck, there should be very little pressure against it. You can actually fret notes without even having your thumb touch the back of the neck at all; try it. If you put a lot of pressure against your thumb, you could injure it.