Amn’t, to quote a buddy of mine, is a good word.
It is entirely underused and underappreciated as far as contractions are concerned. Of course, a lot of its beauty comes from the fact that it is underused and underappreciated–after all, if we used pulchritudinous to describe every other thing, then how magical would it be? The answer: Well, maybe that’s not the best example…
Amn’t’s advantage over ain’t is that it’s a very uncommon word. You run into ain’ts three times before you get to the grocery store, but amn’ts are like good wine in a teetotaler’s cupboard. Its uncommon nature gives it an air of magic and mystery, much like an avocado in New Brunswick, or polar bears in Spain.
Ladies and gentleman, use amn’t if you want to appear bizarre and sophisticated. If you want to get along and appear normal, use ain’t. If you’re wooing a grammarian, I suggest am not, or not doing altogether, because they’re too busy parsing to gerund with you.