It's tough to speculate on what happened with Juggy and Onslaught, since we didn't actually see the 'fight', only the aftermath of him being tossed from Canada to New Jersey (I think it was NJ). Onslaught, (at that stage), was at least at the level of Thanos, and logically Thanos can probably kick the crap out of Juggernaut.
So no, the fight probably wouldn't have gone any different.
Onslaught to Juggernaut is as Juggernaut is to Bane.
Magneto and Xavier working together can beat Juggs (helmet made of metal + psychic attack once the helmet is removed). Combine them into one entity (+Franklin Richards and X-Man?) and Juggernaut stands very little chance if any in my opinion.