This is a particularly critical call when you're purchasing a mountain bicycle. There are really 2 answers to the problem of disc brakes or edge brakes. If you'd like better, more consistent brake performance in all conditions, disc brakes are what you should be selecting. On the other hand, if you would like the lightest set up you may have and you are prepared to accept tiny variances in brake performance, or you desire the best price possible edge brakes are what you should be selecting.

Over time, off-road bikes have gone through many design changes. They started with the first cantilever brakes, then went through the U Brake years, and are now with V Brakes. In most conditions, the V Brakes appear to work best. In wet or muddy conditions, edge brakes will perform poorly.

Over time, they can wear right thru the side of your edge, causing the side of the edge to blow right off. Disc brakes on the other hand have been about for a while in vehicles but were not used on bikes much till the late 1990's. There were some issues in the earlier models, though the cable induced or hydraulic brakes of today appear to work well. Vis performance, disc brakes appear to work better than edge brakes, particularly in wet or muddy areas. Disc brakes usually need less force to apply and aren't effected by the edge or wheel condition. Cost is a problem as disk brake systems have a tendency to be costlier than edge brakes. Mechanical or cable induced brakes are a closer match, though they may still cost more.

Hydraulic brakes on the other hand cost a load more. When you make that last choice, weight out the above options then make your call. Some riders prefer disc brakes, while others like edge brakes - making it a matter of standpoint.