I don't think anything would have been different. Buffy would have continued to shut him out, he would have continued to fall apart because of it. Riley was barely on Buffy's radar in season 5, I can't see a single thing that would have been different because that's how unimportant he was to her life. More than likely he would have gotten himself killed trying to save Buffy in The Gift.
On the contrary, I think that there actually would have been a major difference. I agree that Buffy had begun to shut Riley out, and the more that she did so, the greater his dependence on vampire prostitutes would have become, until, instead of any kind of asset to the Slayer, he became a complete liability. I would go as far as to say that, if Riley had stayed in Sunnydale, he might very well have died there, killed by his addiction, the same as almost every addict, unless they are able to break away from their addiction, is. He might have even, ultimately, wound up becoming a vampire. That would have been the kind of plot twist that Joss would have loved.
Also, if Riley had stayed, there would have inevitably wound up being a confrontation concerning Spike, which would have almost certainly ended quite similarly to what did actually happen later in Season 7 in "Lies My Parents Told Me," with Buffy making pretty much the same decision for exactly the same reason. The less she needed Riley, the more Buffy came to depend on Spike, especially once Glory showed up, for the very reason that she gave Giles. "Besides me, he's the only one who has a chance of protecting Dawn from Glory. He stays. Get over it."
Day 5. 25 days to go.
Greater love hath no man than that he would give up his Spuffy for a friend.