The rules are simple. Two candidates from which to choose to represent the Democratic Party in the race for office of the President of the United States: US Senator Hillary Clinton or US Senator Barack Obama. No write-ins, no declarations of "neither". Only those two candidates, and if you respond you must choose one.
The question you need to ask yourself is this -- do you think that either Obama or Hillary can beat John McCain in the fall? You can take my answer with a grain of salt because I'm a Republican, but I don't think that either one can, in part because the Democrat primary was such a softball campaign. While Obama and Hillary might have been affraid to go negative with each other (and howled in outrage at any sign of negativity), the general election isn't going to be so pleasant and the race card and sex card is only going to play so well to the middle, especially if it's seen as evasive. Getting hit by a barrage of attack ads after looking untouchable is far worse than a trickle of attack ads that get people used to seing negative things said about the candidate.
And while Obama may not seem divisive now, he's going to be asked some pointed questions before this is over about divisive issues like affirmative action, slavery reparations, dealing with the Middle East, and so on that are going to be divisive, even if he declines to be specific. It's easy for him to play to the Democrat base but it's going to be a lot harder to play to the center, which is what Hillary has been doing with her eye on the general election, probably to her detriment in the primaries. Obama has been running to Hillary's left, which will probably come back to bite him in the general election.
So, if I am correct that neither can beat McCain, the best thing for the Democrat Party to do would be to nominate Hillary. That would serve two purposes. First, it would help discourage her from future bids for the Presidency, leaving room for fresh candidates and, second, it would save Obama for later, when he has a little more experience under his belt and another primary season to test him better. And if I'm wrong and one can beat McCain, Hillary's chances are probably better, not only because black voters already vote 90% Democrat but women are more of a toss-up but also because she's been more consistenty centrist, keeping her eyes on the general election. Things that Obama has been saying may appeal to the Democrat base to get him the nomination, but they'll make for great commercials for his opponent and probably won's appeal to the center nearly as well, and whoever the Democrats nominate are going to have to fight McCain for the center.
Why don't I think either candidate will get elected? A few reasons. First is that McCain owns the center an it going to make it difficult for either candidate to get votes there. He's also going to be difficult to paint with the Bush brush since he's stood up to Bush enough on various issues. I also think that Democrats confuse Bush's low approval rates with the idea that voters are looking for far-left proposals like national healthcare and so on, which isn't necessarily true. I also doubt that leaving Iraq will be as appealing to the center once various people start framing that as cut-and-run and McCain, by virtue of his Vietnam War credentials, can be very credible on that issue.
But perhaps a bigger issue will lie in the Democrats themselves. Various Democrat factions have a great deal of emotion and hope invested in either Hillary or Obama and I don't see that easily transferring to the other candidate, especially if this reace comes down to the convention. And while a Hillary-Obama or Obama-Hillary ticket seems plausible, I don't think it's going to happen. A Presidential candidate doesn't want a VP candidate that will eclipse them when it comes to attention (Hillary asking Obama) or that comes with a lot of baggage (Obama nominating Hillary). So while there is plenty of talk about the Republican base staying home for McCain, I suspect a sizable number of Democrats will be disillusioned by the process when their person doesn't get the nomination. And priming their base to believe in stolen or rigged elections could backfire if they believe that, for example, Hillary won because the fix was in.
With a little more experience and a meatgrinder primary that actually test his ability to deal with negative attack ads and tough questions, I think Obama could be an excellent candidate but the puffball primaries this year didn't set him up for a win. He needs to be tested, not adored.
If you do think that either Hillary or Obama can win in November, the best thing you can do is vote for the one who you think stands the best chance of winning, because from a practical policy standpoint, there isn't a huge difference between them.