I know your talking saltwater. But I had a 750 in my 75g and it was horrible IMO
the water movement and flow was just awful. I ended up buying I think it was the 1050 and I'm much much happier.
Anyway. The flow for your 55 saltwater will depend on what you wanna keep also. You may have corals that prefer high to medium flow rather then coral that only need low to moderate flow. After knowing how you wanna stock it then I think you can make a better decision. Do you want flow? Of course... I'm not sure you'll be happy with one however. What are you using as a filter? That will cause flow as well...
Koralia has a chart and will tell you the gph
of each power head. I think and again again this is my opinion but I think they under rate the flow of the power heads. I have a bio cube 29 with 2 Koralia 425's and an upgraded pump from stock to the MJ1200 and I love the flow, I also have the power heads alternate every hour, I use the Hydor smart wave.