First you have to consider what you want your robot to do.
Do you want all wheel drive?
If you use a toughbox mounted mid-chassis to each side you'd have to use chain to transmit power to each wheel(Kit Bot), you could direct drive each wheel(1 Toughbox/1 Motor Per Wheel) then each wheel could be powered independently of the other.
Our Lunacy robot utilized direct drive and we could push 2 robots across the field last year.
We're doing the same thing this year to avoid chain, and keep our ground clearence high and CG low. Since each wheel/trans is spread out on the corners of the robot.
If you equip a toughbox with 2 motors then you have doubled the power output.
Its not really a question of which is better, its all about what you want you're drivetrain to do.