With 2 96s on a 55, you can keep soft corals, LPS, shrooms, polyps, and not much else. Four 96w bulbs will allow you to also keep many SPS, and derasa or squamosa clams. If you could cram one or two more in there, IMO you can keep anything you want.
However, I would still recommend MH if you don't want to be limited on what you can keep. You will save a lot of money in the long run if you get MH to start out. What do you want to keep though?