Keep the V-brakes. Get Kool-Stop "Salmon" brake pads, and your brakes properly adjusted. You will be stopping quick enough. I used to mountain bike, rims and brakes covered in mud, never an issue stopping (way before I finally upgraded to Kool-stop...last year).

If your going to put ONE disc-brake on a bike, put it on the front. But I don't see a need for them for 99% of riders. We never used to have them, and people stopped just fine.