I think it matters hugely which one is elected. Not being an American I don't have to make any run-down of the issues or try to predict in detail how the two would act in office. Of course the financial turmoil is likely to change the conditions too.
But McCain is a military man in his outlook, the Vietnam war and his captivity was the defining experience of his life. If a conflict between the US and an "enemy country" or "rogue state" is knotty, he goes for a military solution, open or veiled (gunboat diplomacy). He argues by posing ultimatums and mowing the enemy down, or trying to. That's what he proposes to do in Iraq (the 'success' of the surge is partly delusory I think, it depends on frail truces to keep some of the militias calm, plus any attack on Iran would make Iraq flare up again), to North Korea, to Russia in a crisis, to deter Iran from getting nuclear weapons - to any enemy. That's really just a continuation of Bush, father and son, and it ignores that sometimes this kind of attack mode will just make the problem much worse. We've seen enough of this "me against everyone who's evil" atttitude under Bush, and many NATO countries are fed up with it.
Now that the US is already carrying on full-scale war in two theatres in the Middle East and some are looking to another war effort in Darfur or Iran, while the military spending is weighing the economy down, there's a dire need for more multi-partner solutions, not world policing by one country. And peaceful or diplomatic solutions. You don't build a free country by exposing it to war and trying to bring on some elections afterwards. Democarcy often takes time and peaceful efforts.
Obama clearly understands the need for engaging more countries as allies, more on their own terms, and to build understanding, not sectarian resistance and despair (in that, and in his overall secular outlook, he's just like the Kennedy brothers).. He wants to keep the US in the forefront of course but he realizes it can't be done by some kind of Roman Imperial style where you act as if America always was the chosen country. That's probably a hard bullet for many Americans but the time when you could count on just one power dictating everything is rapidly slipping away.
Obama seems to grasp so much clearer, too, that political change and new openings in life comes from below, from the ordinary people, not from just following your leader and his scheme. However much he's been jibed for having been a community organizer by McCain's campaign people, it's the kind of work where you have to engage people and listen to their needs, instead of trying to march them into your own design. Where McCain only thinks in terms of encouraging single acts of heroism and endurance to survive bad conditions, Obama sees that you may have to empower those city people to get their trust and make them stand up.