I think the Big "S" had disc brakes out in the 1970's although not for the off-road Market. The first discs I remember seeing were Prostops or early Hopes and that must have been early 1990's, but it wasn't until the mid-90's that they really took off (certainly for XC use) imo.
I'm just about to fit a disc brake to the front of one of my retrobikes, it still has a canti brake at the rear though. They are a bit heavier than a rim brake setup generally but there really is very little in it, the big difference is in the stopping power, especially in the wet, and I think that is well worth it.
Personally I favour hydraulic discs over cable ones, not had much experience of cable discs though.
Oh, you do need a disc compatible hub though, which can make the switch to discs a bit costlier.