Here is a more accurate (but somewhat more complicated) kinetic energy calculator: http://www.teamcosmos.com/ke/ke.shtml. In the KE energy equation energy goes up with the square of the speed, so its most efficient to keep the mass relatively low and increase speed to get more energy. You can also get more energy for the same weight by moving the weight out to the edge of the disk, which is why most disks have a heavy rim and spokes.

What motor you use depends on your budget and how destructive you want the bot to be. Historically, bots with an Etek (now replaced by Montenergy motors) have always been the most destructive.