Either combination would be the best thing to ever happen to this country: Clinton/Obama or Obama/Clinton. But, it's about numbers. Hillary Clinton is 60 years old. If she were the VP, she'd be 68 after Obama's two terms.
Obama is 44. After eight years as Clinton's VP, there would be nothing stopping him from having his own eight years as President. He'd only be 57!
We could be looking at Democrats in the White House for 16 years!