Hmmm.
1. Althought the orcs were intially scared of the Riders, I'm sure it wouldn't take them that long to realize that there was-what?- less than a hundred of them left? Eventually the Riders would've been cut down if the Uruks didn't flee up the steep hill that Gandalf wasn't there to come down.
2.
If the Uruks did flee, they would've escaped the Riders & regrouped, sooner or later the Riders would've fallen.
3. If the Uruk-Hai would've run from the Riders into the Trees, yeah, the Riders would've won-but-the only reason the Uruks went there in the books was because Gandalf & co. were stopping the other gap.
My opinion is that if the Uruks would've stood & fought, the Riders would've gone down, if the Uruks would've fled, the Riders would eventually have gone down. In other words (
): I think the Uruk-Hais would've won.