I think the Democrats will keep Obama in as their candidate. That is unless he continues this seemingly reluctant move to the center that he has recently started to make. I really don't see the Democratic party trying to replace him other than that.
To answer your question I personally would vote for Hillary Clinton. She has more experience when it comes to public affairs both domestic and foreign. She is a former first lady, so she knows a thing or two about the goings on in a presidency.
I have heard her mention that the office she now hold as Secretary of State will be her last endeavor into politics. I also remember hearing her say that she wouldn't seek the nomination as the democratic parties candidate in 2008 so I guess we will have to wait and see.