The rule around here, and one that I agree with, is to get the biggest tank that fits. So if you are considering those sizes, get the 6x2x2. It also depends on what size the aro is when you get it. If you get a baby, you can get away with a 4x2x2 for maybe as long as a year. But if you can fit a 6x2x2, you should. Then you'll be able to keep him in there for much longer without being forced to upgrade. 6x2x2 isn't ideal for an older aro, but it's not absolute torture either.
As for tankmates, it depends on what aro you get.