[thefro526]

Assuming that you are using off the shelf components a Mecanum drive is easier to make. Omni-Drives require precise angles (60° for a Kiwi and 90° for a Holonomic) and if you are not near perfect your robot will have some strange driving characteristics. Also it is much easier to climb ramps with Mecanums. To my understanding Mecanums are also more efficient in forward and reverse directions (like a normal wheel) while still giving you lateral mobility. Omni-Drives have a much lower efficiency (~50%) in all directions but they will give you true omnidirectional motion. Also Omni-Drives require much more skill to drive than a Mecanum Drive. Still, you should test out both drives to see what works better for your application and to suit the game.