and the only issue was “Who would you rather work with?” who would win?
Probably Hillary. The Senate is not known for radical action, and Hillary would be the safe choice. She does have a proven track record of working with both sides.
Although you never know. With Lieberman backing McCain, maybe one or two other conservative Dems could throw it his way.
At any rate, I suspect Obama would be the last choice, and it would go either to Hillary or McCain.
McCain. Hillary and Obama would split (not evenly, I suppose) the Dems, and the Pubbies would unite behind McCain.
Having worked in the Senate, I can tell you with certainty that in a secret ballot McCain would get maybe a dozen or so votes from his fellow GOP Senators. Most either dislike him mildly or hate his guts. He’s a self-righteous grandstander who has a violent temper and doesn’t work well with others. He’s pretty thoroughly disliked among his colleagues.
In reality, if senators elected the President, those on each side of the aisle would caucus together, then vote together for their agreed-on candidate. So, since the Dems have a (bare) majority, and they are likely to prefer Clinton, I suspect that Clinton would get the job.
But this is extremely hypothetical, because this would change the whole dynamic of presidential politics. Those wishing to become president would aim to become senators as a stepping stone to the White House. In addition, elections for senators would become more like elections in a parliamentary system: people’s votes would be based (in part) on which candidate a senator would vote for a president.
But they need Lieberman for that majority, and he has endorsed McCain. I think he’d break rank and throw it to a tie (at best).
A solidly-Dem Senate would be such a nice thing to have, if only to consign Lieberman to the whiny irrelevance he so desperately deserves.
They’ll have that in January unless something really bizarre happens.
Yes. Something as bizarre, as utterly unlikely, as the Patriots losing the Super Bowl.
weeps softly
No, seriously, I’m counting not a single Democratic chicken till they’re all safely across the road. 
I know it’s hypothetical. I raised the question because of my suspicion that the way the public perceives McCain and Clinton (the former decent and honest, the latter not) does not match how their colleagues think of them.
The notion of McCain as an ethical, straight-shooting, honest maverick is one of the biggest snow jobs ever foisted on the electorate.
Renob, in a non-partisan election (without party preferences or caucus), who would be the favorite “son” candidate from the Senate. And don’t restrict it to just the three current Senators running. Is there someone else out there that is known as just a swell guy/gal?