Most rating systems tend to map a 400-500 point rating difference to a near 99% expectation of the result. With that being said, I think a 1200 denying a 2000 a win is more monumental (unless the 1200 botched the win! :))
I'd use the ELO/USCF/Glicko rating gain or loss after both games to make that call.
In the context of tournment stats, is this alway a win, or could it be a draw?
Eg, is a 1200 drawing a 2000 a bigger upset than a 1200 beating a 1300?