Most brakes are usually powerful enough on a bike to lock or at least throw the rider over the handle bars.
Theres only so much force you can apply using the brakes anyway before other forces come into play.
I always think you have to weigh up the advantages of each system before making a choice...
V Brakes
Advantages - Large Wheel circumfrance / Rim. Light weight, Cheaper pads. Cheaper to upgrade
Disadvantages - Affected by weather conditions, fiddly to setup correctly and require adjustment every now and then to get the best from them, cause's rim wear
Cable Disk Brakes
Advantages - Easy setup and adjustment, requires very little adjustment after inital setup. Not affected be the weather or mud
Disadvantages - Slight increase in weight over v brake, cheaper disc setups are generally weaker than the same price v-brake.
Hydro Disk Brakes
Advantages - Adjustment is automatic, very very powerful setup. not affected by the weather or road conditions
Disadvantages - Slight increase in weight over cable disk, expensive, Can bind and would require periodic maintenace to ensure pistons do not stick. Expensive if master cyclinder needs replacement
Disks are better on a bike application. If I was able to spec a bike I would choose a cable disk brake. Not so fussed on the rear being a disk would be happy with a v on the rear. However currently I have V brakes all round which do the job very well. Even though there a budget brake system.