Personally I like Disc brakes off road, but then again I try to avoid excessively sloppy trails and holes (working on a muddy truck sucks). I think disc brakes are faster/easier to dry off after a creek crossing, not to mention you dont have to worry about the adjusters and springs rusting up like with drums. Another problem with drums is that they tend to build up some nasty muck inside them rendering them useless. But my buddy swears by his drums. He ran discs for a while on the back of his BII (dedicated trail rig) and said they didnt last. with calipers all you have to worry about is the caliper pins (as long as the boots are in good shape and you lube the pins they arent an issue)
Just remember that if you run an after market disc setup, then you also have to buy after market replacement parts. After all brakes are a wear item and WILL need to replaced. Wouldnt it be a lot nicer to walk into a parts store and just ask for some 98 Exploder rotors and pads and them have everything in stock. With after market parts you have order things, pay after market price

, and not to mention its pretty common for after market companies to discontinue product lines and parts.
-PlumCrazy