Short pants and tights are more in fashion? (Robert Curthose)
More seriously, looking at the Wiki article on him, he looks like an A**hole, even for a Plantagenet.
Earlier Magna Carta?
He wasn't a Plantagenet; they were descended from Robert's niece Matilda and her husband Geoffrey (Plantagent) of Anjou. He definitely was a bit of an asshole though. IIRC, his brothers hated him, and he and his father didn't care much for one another.
Anyways, I would expect that if William wanted to give Robert England, he would still split England and the duchy of Normandy -- Normandy would just go to William Rufus. Which would complicate issues for Curthose in ruling England, obviously - as I said above Robert was hated by his brothers, and Normandy is a powerful base from which William or Henry could mount a rebellion from. Obviously it didn't work out for Robert in real life, but William (or Henry) might have proven more lucky.
If we're going with the OP's scenario that Curthose gets the entire inheritance, I feel like there would end up being a lot of pressure for Curthose to at least give some land to William and Henry. Maybe he splits Normandy between the two of them, that way they're both pleased and pit against each other rather than at Robert. So things probably work out better for Robert; more patronage power is always good. Unless Robert was really stupid and held on to everything at all costs -- that's a good way to find yourself on the wrong end of an assassination conspiracy.
Maybe the biggest point of interest is that Curthose joined the First Crusade as the Duke of Normandy. Perhaps being king changes that decision making process (I don't know enough to determine that), but I think things are more interesting if Curthose becomes the only king to join the First Crusade... could be a big impact.