I've used cantis, v's and discs on a tourer.
Maybe I've been lucky, but by far and away the least maintenance intensive have been the v brakes -put in a pair of Koolstop salmons and you are well away (as is the case with cantis too). Brake pads being equal, the v's always seemed to offer more power and simplicity in setup than the cantis. The advantage with cantis is that you can use road brifters and have fender clearance -v-brakes you need converters which seem to be fiddly (at least from what I saw of my friends).
Discs...discs.... well, I love 'em. I have Avid road bike discs, and they stop really nicely with excellent feel to them. Disadvantages? Well:
i. it's true that I had to fashion my own front fender and rear rack bracket to get them over the disc brakes
ii. heavier, harder to setup just right (I don't wany *any* squeal)
iii. more expensive
iv. break down in outer Mongolia, spares are hard to come by. Course, you have to ask, will you be in outer Mongolia doing a tour, or closer to some LBS?
Another disadvantage with discs is that you really really don't need them, they just provide better performance. Well setup v's or cantis are usually more than adequate, truth be told. But then again of course, people used to tour with 3 speeds and pull rod brakes, so take that for what its worth. Let's just say I can't see a reason to take them off my tourer, and I love having them on it.