Well, a 55g is 4 feet long. So, the question becomes this: Are you going to use two filters, one on each end, or are you going to try to use one filter to circulate water from one end to the other? On 4 foot tanks, many people opt to use two smaller filters instead of one larger filter. It boils down to a matter of preference. If you are going to try to use one filter to circulate water across 4 feet and then back again, I would go higher than the 303. With two filters, you could circulate your water well with even currents instead of having a super strong current in one area and a weaker current in the other.
"Insanity: doing [or asking] the same thing over and over again and expecting different results."