Well, I would think a 280 would be sufficient in a moderately stocked 55 gallon. If you had cichlids or otherwise heavy fish load, I'd suggest 2 280's,,, for redundancy
That being said
I also agree that the Aquaclear 300 is a bit better at mechanical filtration than the 280... the sponge real estate is a bit bigger but you don't get as much biological (no wheel).
In the end both are very good filters and either one would be fine for the 55
I guess the next question is, what is going in this thing? plants? tetras? massive cichlids?