On tonight, I don't foresee a decisive win by either. It will come down a lot to what questions come up, but both are capable of going back and forth while while still coming across as likable.
I agree with Kaylore, that Biden probably shouldn't come out attacking. I think like he and Clinton did at the DNC, he should make a case for the Obama admin's accomplishments.
As much as the GOP attacks that record (which is their job when running against an incumbent), the DNC showed that a well delivered defense could move the polls. Not addressing his record (like in Denver) sends a bad message, and allows the Romney narrative of the last 4 years to stand unopposed. I think Biden will work to get back on track with that as priority 1a, and attacking Romney/Ryan on Medicare and taxes as priority 1b.