This can be a very important decision when you are buying a mountain bike. There are actually two answers to the question of disc brakes or rim brakes.
If you want better, more consistent brake performance in all conditions, disc brakes are what you should be choosing. On the other hand, if you want the lightest set up you can have and you are willing to accept small variances in brake performance, or you want the lowest price possible, rim brakes are what you should be choosing.
Over the years, mountain bikes have gone through many design changes. They started out with the original cantilever brakes, then went through the U Brake years, and are now with V Brakes. In most conditions, the V Brakes seem to work well.
In wet or muddy conditions, rim brakes will perform poorly. Over time, they can wear right through the side of your rim, causing the side of the rim to blow right off.
Disc brakes on the other hand have been around for a long time in cars but weren`t used on bikes much until the late 1990s. There were some issues in the earlier models, although the cable-actuated or hydraulic brakes of today seem to work quite well.
In terms of performance, disc brakes seem to work better than rim brakes, especially in wet or muddy areas. Disc brakes normally require less force to apply and aren`t affected by the rim or wheel condition.
Cost is an issue, as disk brake systems tend to be more expensive than rim brakes. Mechanical or cable-actuated brakes are a closer match, although they will still cost more. Hydraulic brakes on the other hand cost a lot more.
When you make that final choice, weight out the above options then make your decision. Some riders prefer disc brakes, while others prefer rim brakes – making it a matter of opinion.