I would put two on a 55. The older ones are better, they can be ID'ed by the lid which has a hole to pull the lifter thru from the middle rear of the filter body. On the filter body make sure the lifter pull is there, the new ones don't have a pull and the lid is solid in the middle rear. Although I have had my issues, I would still try this filter again, keeping in mind it may have a few issues.
I never had any problems with the older ones, and when I sold my tanks back in 2004 or so, the filters went with them. I bought two new tanks this years finally, and two Emperor 400's. That's when I noticed the changes that were made, and the impeller was really grinding a lot and noisy - I could hear it 30 feet away. I used to fix that by adjusting the lifter pull, but now it didn't have that. I tried many things to fix it, none worked. I contacted Marineland at that point, they sent me a replacement for each, and the replacements had the same issue. More items were shipped, one even arrived with an impeller fin completely broken off - I found that when I disassembled the filter to inspect it before using it. Quality control is slipping. I won't go into much more, aside from saying they shipped 5 boxes out to me and I returned one very large box back to them. In the end, I accepted two Penguin 350 filters that run quiet and have bio-wheels, with a large supply of filters and media cartridges. They declined return on the filter with the broken impeller, so in the future I may buy a new impeller and mess with that.