Oh I understood the definitions but, to me, making a difference between the two is pointless. In both cases you use the Harry Potter canon since you always use the characters and, very often, their personalities and physical traits. The only difference is your starting point. With AR, you usually ignore all of the events happening in the books, or at least most of them, although some writers tend to skillfully integrate bits and pieces of it. With AU, you don't. To me, it's pretty much just a matter of branching out earlier rather than later.
I got that. I just think it's going to be difficult to define some criteria to define what exactly is 'best'. Unless it's left to subjectivity in which case the question is moot.