I think Ascheck is right in that the efficiency and power between the Castle-Neu and Medusa 80mm will be probably very similar. The Medusa will have a *slightly* longer rotor, but the difference will be very small. All I know is that my Castle motor stays ice-cool (not literally lol...but hasn't hit over 130F).
However, the Castle motor is built much better for offroad use, IMO. Bigger bearings, and a much thicker/tougher casing. That the windings terminate inside the motor and are presoldered to nice, thick 10AWG wires makes things even easier. It's also sealed, so you won't get any rubbish inside. A word of warning though; The Castle motor on 6S will be utterly incredible! Good job you're bashing, as it'd be way too much for racing unless you have exceptional finger control.
Either of those motors can produce insane amounts of power and remain cool, but I'd go for the Castle personally. It's just built better.