The Muggy is a great basher aswell and comes standard with 25mm diameter shocks....
I don't know anything about the Mantis. But I do know reading from others on this forum that the Muggy/LST drivetrain is one of the toughest and will take any powersystem you throw at it. My Muggy will soon have a NEU 1521. It is a heavy truck so you'll probably need more power than the Mantis for the same speeds.