Miley came from the star-making machine that is Disney Channel. Hannah Montana was huge and then she capitalized off of it with the Meet Miley Cyrus album. Years later when it came time to break away from the Disney-image she had to do some drastic things and that's where all the antics & gimmicks came from. The controversy helped propel the music. The true test will be to see how she does in her next era.
Ariana came from Nick which isn't as good at making stars, so her success has been very impressive. I mean, just look at what happened to Miranda Cosgrove & Victoria Justice. She also didn't really have to do much to separate herself from the Nick because she wasn't the star of the show, so there was no controversy to help her sell.
Miley has a good voice, but Ariana has a great one. Miley has better stage presence though. Although, Ariana dances while Miley really doesn't. I don't think the GP paid much attention to Miley's music besides the hits until Bangerz came along, while they cared for Ariana since her debut with The Way.
Miley is bigger because she is a celebrity. Right now Ariana is a singer, but she's entering celebrity status.
Mess at Miley stans & Ariana stans fighting anyway considering they just did a collab. When we should be coming together to drag Demi, Selena, & Taylor.
only assumptions based on personal preference 
2 million > 1.6 million
These ari stans need to realize that licking donuts won't keep their fave relevant for long!