In my opinion, they are just a better class of HOB. Smaller, quieter, more efficient, more room for a greater variety of media...the list goes on.what is the benefit of the AC that I am missing out on?
I used to praise Penguins and tell everyone to get one, and I still do to a certain degree, but once you have tried AquaClear filters, there really is no comparison.
I'll still use my Penguins on none planted tanks with smaller bio loads, but for anything else it will be AquaClear.
And always, if you can afford a canister filter, get one.