Even DC motors `convert' DC to AC -- they just do it mechanically, via their brushes and commutators. I'm guessing that the AC motors you're referring to are also called brushless motors?
If so, they're good because you don't have any losses in the brushes and the ESC can do things like adjust the timing of the motor on the fly to make it more efficient. Ultimately, brushless motors tend to be more efficient than brushed motors. It's not a huge difference for good quality motors, but it's significant.