I think it'll be Hillary.
You can pretend all you want that it's true that race doesn't play a part and we're all knowledgeable citizens whose only concern are the issues, but Obama is the first real shot of having a "black" man elected President and calling him a "black" man proves my point. He's multinational but most people judge his race based upon his skin color. He was born in Hawaii but no one calls him a Hawaiian.
Hillary is the first real shot of having a woman elected President.
There are plenty of Democrats that want a woman in office - but not a black man and there are plenty of Democrats what want a black man in office but not a woman - so even when a nominee is finally chosen, it doesn't mean that the nominee will gain the others supporters.
There are tons of people that don't want a black man or a woman in office - which gives John McCain an incredible edge. He can almost keep his mouth shut and let Obama and Hillary kill each other and win over those Democrats whose nominee wasn't chosen over to his side all but ensuring his being elected the next President of the United States.
A major way to counter that is for Hillary or Obama to choose the other as their VP and keep those votes on the side of the Democratic ticket.
Even though their candidate didn't win the Democratic nomination, the person they supported would be the VP and the front runner for President in a future election - keeping the dream alive that their "race" or "gender" would eventually take office.