The friendliest place on the web for anyone with an interest in aquariums or fish keeping!
If you have answers, please help by responding to the unanswered posts.
I don't have either, but I just got two 404B's. I like them, but I measured their flow with no media at all (just hoses in a sink/buckets) at 278 gph vs. the rated 525 gph. Perhaps others have more data, but I'd assume their specs are exaggerated (probably all manufacturers do -- I know my Fluval 306 is WAY lower flow than I anticipated), and go bigger to be safe.
I would get two of them, I've got a single 304-A (no UV) on my 27g cube. It is amazing. Overkill, especially since it still has a hob that handles 50g tanks, but I love it. For a 55g tank try to get a 304/404, they really are not that expensive. just make sure you have room in the stand for them. They are large.