Well, sweetheart, you really are in pretty much the classic tough spot. Definitely the guy from the 16th is a candidate, and even with such an early ultrasound, the guy from the 21st is not possible to absolutely rule out. I would guess that the first guy has the odds, but not, unfortunately, by enough of a margin to bet on it. A DNA test is the only way to go.
I get conception as being on November 19, give or take a day. Was the sex with both guys unprotected? You're going to need a DNA test, I'm afraid.