Good comparison, motor temps are very hard to detect on both motors, the coils are on the inside of the motor on the stator the magnets on the outside so temp transfer is very hard to detect, The BMC and the X5 coils can run very hot indeed it depends on power and usage of course, the BMC copes quite well even at power levels of 1500W or so but problems start to arise very quickly above that, the first thing to suffer on both motors is the hall sensors.
Quite a few people have found the hall sensors are very sensitive to high temps, the new motor is exactly the same as the new Team Hybrid motor, keyed rotor etc.
I have been using the X5 and the BMC for over 2 years now, the X5 is a much more robust unit at higher powers, it handles the higher power much better than the BMC, the gears on the BMC wear much faster on the BMC when you up the power, another thing that can suffer is the main drive bearing on the rotor, this is sadly the weakest part of the motor when being used at high power levels, the combination of heat and power can break the bearing, effectively locking the rotor, turning the wheel backwards with no batteries connected to the controller can diagnose this problem quite quickly.
The gears can be broken on the BMC if you use high power in combination with fast loaded starts, jumping and or hill climbs.
It should be fairly obvious from their physical size difference and build that the X5 motor is much more capable of reliably handling higher power levels than the BMC for sustained periods.
During the clash of the titans meeting we broke 2 BMC motors, one was down mainly to jumping but the other went under acceleration, that was a new 600W motor like this one, using steel gears with a keyed rotor, the motor was being run at 72V 35A, a few hard starts shredded 1 gear and broke the main rotor bearing.
I still like the BMC motor, love its weight and its acceleration however its not really built for the long haul, the X5 is heavy but it is built for the long haul, sure it could do with some improvements but its nearly there.
If the BMC factory actually listened more closely they could build a reliable motor at much higher powers than currently available, the BMC if used sensibly is a great motor, you abuse it and it will break, the X5 is much harder to break and if they fit better halls to them and or utilise a sensorless controller for them ALA Justins controller then I think you have a motor that will last you a very long time indeed.
Just my 2 cents worth as usual