Welcome to AC!
The main difference between the hang-on and in-tank skimmers is that the in-tank skimmer will take up valuable space. If you have room behind the tank, use a HOB model. Someone on this board, I think it's Oakely, uses a Tunze in-tank model if you're curious. You can get good and bad makes of each, so be careful. The AquaC RemoraPro would be a good choice for a 55.
A sump is a separate tank, plumbed to the first, that adds volume to the tank, and allows you to get stuff like the skimmer, heater and the like away from the main tank. It can also hold a refugium, an area where macroalgae and critters can grow and export wastes and/or generate plankton.
Hope this helps.