It all comes down to whether you think States move completely independently or whether a uniform national swing is more likely. Let's see:
If Obama loses Ohio, he needs to carry NH and IA, where he is a modest favorite, as well as at least one of Virginia and Colorado, which are pure tossups.
This is a reasonable possibility if Romney significantly improves in Ohio without making any gain in other States. If you think such a situation is likely, then vote for Obama.
If Obama takes Ohio, on the other hands, Romney needs to make inroads far beyond this. He needs in his bag not only VA and CO, but also IA and NH and, particularly difficult, either Wisconsin or Nevada, where he pretty clearly trails.
This outcome is slightly more plausible if you imagine a national shift of about 3 points for Romney. Obama could still squeak out a razor-thin victory in Ohio, while being caught by surprise in either WI or NV (this, of course, requires that you trust Nevada polls). It would not be the most likely outcome, but it would be fairly conceivable in the event of a national Romney bounce. If you think it's possible, then vote Romney.
The bottom line is: both are
very unlikely.