There will be metaphorical black eyes and bruises as these two take to the big boy playground and duke it out.
At the moment it's in Obama's favor because the economy has tanked and he can pretty much make his point with the redundant "8 more years" mantra.
Also in Obama's favor-- the more he campaigns against a single opponent, the better he gets at strategizing their weaknesses and playing up his strengths-- he was quite effective in doing this with Clinton.
It's all in the approach.
McCain will have national security in his favor-- no matter what.
McCain will paint Obama as an inexperienced.
Visually, Obama has it over McCain-- height matters, and McCain is shrimpy in comparison. McCain will also appear like an old man, regardless of his relative good health. There's no comparison.
I predict Obama will wear a red tie-- it exudes Power. McCain, a George Bush light blue tie.