In the last 7 polls:
McCain leads Clinton by an average of 1.6 points
Obama leads McCain by an average of 3.5 points
In fact, Obama does better than Clinton in every single one of those 7 polls. The more liberal candidate is the more electable one. The choice is obvious.
Yes, I know that RealClearPolitics is a right-wing hack site, but they included most or all of the recent polls together with an average.
Here are the polls:
Rasmussen: McCain beats Clinton 46-43, Obama beats McCain 47-42
Time: McCain and Clinton tie 46-46, Obama beats McCain 48-41
CNN: Clinton beats McCain 50-47, Obama beats McCain 52-44
Cook: McCain beats Clinton 45-41, Obama beats McCain 45-43
ABC: McCain beats Clinton 49-46, Obama beats McCain 49-46
Fox: McCain beats Clinton 45-44, Obama beats McCain 44-43
NPR: McCain beats Clinton 48-45, McCain beats Obama 48-47
Some additional stuff: Of course these polls aren't proof of what will happen 9 months from now, but General election turnout is easier to predict than primary turnout, so they shouldn't be off the way some primary polls were. Also, since most people either supported McCain or the Democrat in both cases, the MOE is actually smaller for comparing how they do.