Generally, the more damage a weapon dishes out, the heavier it is. Heavy components slow Bots down, but they tend to make more of a difference in the speed and maneuverability on the lighter Bots than they do on the Heavyweights. Some engines are balanced all around, while others provide a lot of speed, but not much shielding. Other power plants will give you a lot of protection, but you might have hard time moving around and jumping. The right power plant depends a lot on how you like to play, and what the game objectives are.
Bots is a one on one game.