I bought the Reign and so far loving it, not ridden the RM so cant compare sorry. Can say the Reign loves being pointed downhill, will soak up anything you throw at it, not the most nimble bike though. Power wise it is fine, couldn't see myself needing any more and this is coming from someone suffering from Chronic Fatigue (I ride in Trail mode most the time).
FYI, rumors are the Reign is going to be upgraded to a 625wh battery in the very near future. You can also get a 250wh battery extended for the Reign that bolts on where the water bottle sits.