Superskinny.
Even if the reason for being superskinny was due to some eating disorder, I'm judging it solely on the basis of what you're left with when you eventually get back to normal weight. With being supersized, you could end up with a lot of excess skin that needs surgery and such to remove. With being super skinny, you may get a few stretch marks, but they soon go.
Obviously, I'd rather not be either, but between the two, superskinny.