I have 2 AC-500s, and 1 Emperor 400. I personally prefer the ACs because they are more flexible in my opinion as far as media options, and you can go completely with sponge, which is what a lot of AC owners do. The Emperor folks want you to repeatedly buy more of their filter media. They work wonderfully if you don't mind doing that. But there are many people out there I know who hold a completely opposite view on this, so just consider this 1 opinion. :dance