Having been involved in debate in high school, that is the sort I am used to. In such debates (and other debates that I am familiar with) the "winner" it is determined by a judge. As far as I know there is no judge for this type of debate...
I guess one could say "if you were the judge..." but that presents some problems. I never judged debates myself. There's generally a certain amount of experience requisite for that. Aside from that though, in high school debate, I remember that judges for events are never from the same school as any of the students in the event. That way the judge is neutral. People who have already made up their minds who they are going to vote for are pretty likely to pick the candidate they are going to vote for. If you put up a poll that said "If you live in the U.S. and are planning to vote in the upcoming election: who would you vote for?" the results would, I'm almost sure, be very close to the results from a poll that said "If you live in the U.S. and are planning to vote in the upcoming election: who won the debate?" A Bush/Kerry supporter is not going to usually admit, "well, Bush/Kerry lost the debate, but is really still the better candidate."
That said, if I were for some reason the judge of this debate (and I think I've already made it clear that I feel I am unqualified), I would give the win to Kerry. I think that I'd generally anticipate that outcome, because Bush is (for a president) a poor speaker (and I think even Bush supporters agree that is not his strength).
My reasoning though is...
Both men sidetracked and generally wasted time (something I'm nitpicky about). Bush did this a bit more.
Bush failed to address more of Kerry's contentions than the other way around (this is integral from a judging standpoint in all styles of debate that I'm familiar with).
To conclude, I'm not impressed with the performance of either of them for this debate.