I'm going to roll with MJJ on this one, ultimately because he has more experience manipulating reality than Onslaught has had. They both are high end reality manipulators with no upper limit, the question would ultimately be who was better at it, and who had better showings, and that is why I think MJJ would win.
The biggest reality alternation feat Onslaught did was create a sun. If he had more time to evolve and learn to use his abilities I think that he could take a win potentially.
Why doesn't it count? Since original post didn't specify, I went by the last incarnation of Onslaught, where he was capable of creating another sun within the solar system that housed Earth.
Gender: Male Location: somewhere within time & space
Jaspers' Warp was only able to reach a Universal scale because he was stopped
by his (Plot Device) Fury,
before his Warp could spread further into the entire Omniverse.
That's his powerset, it's not a matter of choice,
Jaspers' Warp automatically expands exponentially across all Realities if left unchecked.
BTW. Jaspers was equal to or more powerful than Merlyn.
Merlyn has Omniversal feats.
So it doesn't always come down to who has a greater scale of influence,
because apparently, one can be all powerful within the range of influence one controls.
Gender: Male Location: +40° 36' 5.70", -73° 57' 49.46
I would argue that since their is no stipulations,and the match starts even and fair....Onslaughts Telepathy is the winner here,i have seen enough threads that have the quickdraw style and winners are apparent seeing their powerset.
Jaspers-Reality control universal(omniversle)....
Onslaught-reality control(planetary),Telepathy,Telekinesis,Magnetic controll, Psi Armor,Astral projection and many powers that acompany psionics...mental control,intangibility(phasing)precognition, etc...
All Onslaught needs to do is make Jaspers think he won and leave the field of battle on his own...and many other ways to deafeat jaspers...this is not a fight to the death
But then again, Mr. Master, although I agree that MJJ would take this, but still, shouldn't be Franklin Richards potential greater than MJJ, since he is an omega mutant and MJJ is not.
About FR at full potential being equal to a Celestial, I really think that is underestimating FR power, he should go far and far beyond Celestial.
Jasper's doesn't need to be labeled as Omega to be as powerful as one, if not moreso. That label is pretty meaningless. Elixir is an Omega and I'd take many "Alpha" level mutants over him in a fight. Vulcan is a supposed beyond omega mutant and he got chin checked by Gladiator.
Jasper's would pwn all 3 (Vulcan, Elixir, Gladiator) simultaneously with little effort. You could up the stakes and Jaspers would still come out on top.