In one word: Electability.
I think Hillary would be a great president. I think Obama would be a great president. She definitely has more experience. But she would also bring out the Hillary-haters.
If Romney were the Rebulican nominee then Obama or even Hillary would be a shoe-in. But b/c it's likely to be McCain, it makes the race that much tighter.
While I still think it's the Democrats' general election to lose, I want someone with the best chance of winning and that's Obama.
Don't see much difference in policy and philosophy really, just electability.