Zoo Med 501 or the Tom Aquatics Mini Can - want/need to slow the flow? Pack the canister with some filter floss...
Here is my take on them.
* Hoses are more flexible - On the Zoo Med 501 they don't use those rigid, not as flexible ones like on the Tom model
* slower/less flow on the Zoo Med 501 with empty media chambers - you can adjust the flow on either of them by adding some floss in the canister - but, this will impact the frequency of cleaning to maintain the rate
* The Tom one comes with a hanger - the Zoo Med it is an accessory
* I like the clamps on the Tom one better - the Zoo Med "sliders" are a bit more of a pain to open
* Both come with nice little spray bars
I ran the Zoo Med on a 6.6, and am currently running the Tom one on the side of my 2.5g, with a decent amount of extra media in there. Why that way - because there is nothing to right of my 2.5, so the filter is hanging there, and it was the one that came with the hanger.
If I had to recommend one, I'd say if you are looking for a "cleaner" looking one - go for the zoo med (clearish/smoke-grey hoses) - and get the hanger or be prepared to place it almost at the same level of the tank - back or sides.
If you want something that feels a bit more industrial - the Tom will do it.
Whatever you do - don't try to put CO2 into those little canisters - I've tried it, and it is noisy! Anyway, the little tanks just get a bit of excel now...and the 6.6 is no more.