Lots of people on here with lots more experience and knowledge than me, so I'll only speak to what I know.
Discs. I agree with Dave 100%, best advancement ever. I'm currently riding a specialized hardtail with hydraulic discs, and I wouldn't give it up for a full suspension with Vbrakes, no way. The only change you'll have to make in your riding is to get used to having the same brakes at the end of the day that you had in the morning.
In my opinion, worth getting a new bike to get them.