chattgirl: It isn't a stupid question, but in order to help need to know a little more info.
Are you going to add a sump to your 55? which would allow you to have an insump PS otherwise you will more than likely want a HOB (Hang on back) PS.
Do you plan on upgrading anytime soon to a larger system. I always plan for the next tank so I don't have to "rebuy" Skimmers and the like - it is a big financial investment so I do like a little growth.