Is there a big difference between the Monster Horsepower and the Monster Max motors?

Yes, huge.

The Monster Horsepower also seems to be more likely to suffer heat stress from what I have seen, usually the magnets in the stator go.

My understanding is that the MonsterMax offers significantly lower resistance in the stator which is what helps provide all the extra power.

Huge, the MAX has lower resistance, runs cooler, better rotors, all around a lot stronger of a motor.

As far as speed not much. But like others pointed out the new max runs cooler, which is a a big difference. The new motor does not fade towards the end of the race.