If you are at the height, or approaching the height, of a bout of bronchitis, it seems unlikely (I'm being generous here) that you will be capable of giving your best to the LSAT. A cold is bad enough. Bronchitis is REAL bad. Waiting til December may hurt your chances at getting into School X, or getting a scholarship at School X--but if you bomb the LSAT you will either a) cancel and retake anyway, b) not cancel, and retake when you see just how badly you did, or c) try to convince yourself you are content not going to the school you really wanted to go to.
But who knows. Maybe you're already on the mend, and you can tough it out. I don't think I could; but I'd be mad having to put it off, that's for sure. I feel for you.