An invert tank is generally easier, but that rule doesnt always apply. If im not mistaken, some sea stars are rather hard to take care of, and can get quite large. Some options are the orange sea star, a few species of brittle star, and maybe a marble sea star (though im not really sure about that). If it were me, id avoid the sea stars for a while until you really get used to the SW scene. You're restricted to only a few animals in a 6gal, invert or not, so look over what you want most. I would imagine a few peppermint shrimp, maybe a few sexy shrimp and/or a blood shrimp or something would be fine, but i really dont command any authority in the SW scene. Most crabs and lobsters are territorial (unless you get mithrax or something) and wouldnt be a great choice for such a small tank. As far as corals, unless you make a custom top with a lot of lighting, you're also gonna be restricted to a few speices; namely polyps and mushrooms. In such a small tank corals are hard to take care of becuase a.) water quality is difficult to regulate, b.) intense lighting will always cause the temperature to fluctuate, and c.) its hard to fit enough lighting onto a tank that small.
I think if you got a perc and a few types of shrimp you'd be sitting pretty. Do a weekly water change of about 20%, possibly make a new light fixture (you wont really need the eclipse filter system anyways) so you can fit a few PC lights on top, and after a few months you can try your luck at some polyps or mushrooms.
Best of luck, and always remember: patience is a virtue *thumbsups