Based solely on what policies they would pursue, and what kind of president they would be, I would be OK with either Obama or McCain winning, as I agree with some of the policies from both of them, and they both seem like decent guys (McCain’s recent behavior has reduced my respect for him, but, overall, let’s assume he’ll be a decent president).
Nevertheless, if you are on the fence as to whom to vote for, I think you should vote for Obama.
The reason is that if McCain gets elected, it will send a signal, a loud and clear signal, to both the Republicans and the Democrats that, as long as you pick the right candidates and run a sufficiently skilled campaign, you can get elected to power even if you can screw the country when governing (taking it into unnecessary wars, increasing government spending like there’s no tomorrow, damaging the economy, flouting cherished rights like habeas corpus, removing bans against torture, etc)
If the Republicans win, what incentive will they have next time they are in power (White house + Congress) to not screw the country up even more than they have this time?