The main differnce is the front fork: WP 4CS, way softer and in my opinion not good for MX, but for enduro. MXR has a WP AER 48, Battery, motor and maps are the same. EXR has a bigger sprocket to give 10k more topspeed. And the rearwheel is 18inch instead of 19inch on the MXR.
Rear wheel has already been changed to 19 inch. Rear sprocket is no big deal and I am thinking about using it in the 60+ NO Jump class, hence I should be able to get away with smaller front forks. I would buy a 19 MXR if I could find one.