If I was to vote for Hillary Clinton and see her nominated as the Democratic candidate for President, I have the real risk of being triangulated against, of being ignored, and of seeing the cries of the Broderite media run red. These are real risks, and some can be hedged with more (and better) beer, and others will have real costs in either organizing to defeat getting screwed over or getting screwed over. However I can also assume that Hillary Clinton will be a competent technocratic president as that is her entire campaign's premise, and will be responsive to pressure and changes in her incentive structure.
If I am to vote for Barack Obama and see him nominated for the Democratic candidacy, I face significant and unknown uncertainty. He has intentionally decided to put as little meat on the bones of his decision process and policy process instead choosing to run on his personality and life story. He has consistently produced the third strongest policy proposal as evaluated from a progressive pragmatic perspective, and he does not have a significant case history to see if there are enough predictable points in determining his behavior and judgment with the exception of his superior judgment in 2002/2003 about Iraq; although this superior judgment has led to little deviation from his opponent's record in the Senate.
Furthermore, as Ian Welsh has pointed out at the Agonist, he has one very important enemy in Paul Krugman. Krugman fears he is seeing the same basic characteristics that he saw in late 1999 and early 2000 that sent his warning signals off about Bush --- things were not adding up and any response to queries about the missing sums was met by a media and political response and not a policy change. This is a real risk in a sea of uncertainty concerning Barack Obama.
I'll vote for either in the general election, but if my vote vaguely matters in the Pennsylvania Democratic Primary in April, I'll be pulling the lever for Clinton as I can handle risk way better than I can handle uncertainty.