The reason why they have disc brakes on mountainbikes is because they ride in the mud ... and with muddy rims the typical rimbrakes will have reduced efficiency because the rims will be muddy.
Discs are located at the center of your wheel and so are less prone to getting full of mud.
For normal riding, however, there is almost no difference between any of the available braking systems because all of those systems brake better than your tire is able to have grip enough not to slip ... which means that your tire will slip on the road way before your brake will slip on the rim.