I have two Aquaclear 70's and a Fluval 404 running on my 33 gal reef. They are both crammed with biomax and LR rubble.
Never had a problem. A little harder to hide the heater, skimmer and filter intakes but with some creativity it can be done. Keep the impeller and impeller shaft in good shape and they are whisper quiet. 9 tanks, 9 Fluvals, 13 Aquaclears, 1 bedroom apartment. You do the math.
A sump offers you more options ie deep sand bed, refugiums, hiding gear, etc but IME they are not an absolute necessity. A sump is basically just a larger capacity, open-topped canister filter anyways.
And to qualify, my experience is only the 1 tank I started 6 mnths ago, but, knock on wood, everything has been going really well.
http://www.youtube.com/watch?v=82rPEl0_mzo
I also have no experience with larger tanks. The dynamics might be very different.