The way Kasich's going, he won't even make it into the debates. And there's a good chance - a REALLY good chance - that Biden doesn't run. It's obviously unlikely, but we'll see. People are saying it'll likely be Bush v. Clinton because of polling, and facts. Most likely it will be the case.
I most certainly wouldn't underestimate Kasich - he's never lost an election except for 2000. Biden's also up there with Kasich's ability to break divides.
I wouldn't be surprised if Kasich's announcement spike propels him to the debates - it's easier for him to win the nomination than for Biden to win the Democratic nomination.
In this hypothetical, though, who would win?