By the way, Joss Whedon himself answered the question already :
"But perhaps the most important question of the evening was one of the earliest, as Whedon was asked who would win in a fight, Buffy Summers or River Tam? He admitted that this was the one question of the evening for which he wasn't prepared. He first sought to answer the question democratically, conducting an instant poll of the audience; the results were about a fifty-fifty split (my tenuous grasp of journalistic objectivity prevents me from revealing my vote on the matter). After buying sufficient time to consider the question, Whedon finally reasoned that, while Buffy is stronger, River's whole thing is being programmed to kill, so…He trailed off there, but I think that's a pretty clear vote for River. (What? I said my objectivity was tenuous.)"
well, while Buffy is super strong, and is very skilled in martial arts(need to be, as a Vampireslayer)
River has the training in martial arts too, and she is very good too, she migt not have superhuman strenght, she has incredible speed, and precision, and do not forget she to has the skill of mind reading, she can probably get a little more warning of what an opponent might try, due to that. which makes me think she might be able to win