I would use the 55 for a sump if it were me. Remember that the more water volume you have in your system the more stable your system will be.

As for a sump design, there is no set way. Everyone seems to want their sump to do something different for each application. There are plenty of sump designs on the web, and here on the build forums.

There is no set "design" for sumps but most people tend to go for a 3 chamber sump Skimmer chamber, Refugium chamber,return pump....this is by no means a setup for you to follow you could do a 2 chamber sump skimmer and return .....the choice is strictly yours the only thing is that regardless what you decide to do the sump should be large enough to handle excess water in case of a power failure or pump failure. Nothing worse than coming home to wet floors