Here's some more data — a comparison of counts across all three debates for if (at least according to the CNN transcripts):
(if) Obama McCain Debate 1 20 19 Debate 2 37 22 Debate 3 44 14
And for some:
(some) Obama McCain Debate 1 25 6 Debate 2 22 20 Debate 3 34 11
Given a set of observations like this, we could come to several different sorts of conclusions.
Maybe it's a meaningless, random statistical fluctuation. After all, there are lots of words, and people vary randomly in how often they choose different words on different occasions, and the way I've gone about this analysis is likely to turn up some differences that arise purely by chance.
Then again, maybe the difference (between individuals or across occasions) is real, but reflects a stylistic difference in the way messages are framed (e.g. "If we want to do X, we need Y" versus "In order to do X, we need Y"), rather than a difference in the underlying distribution of messages. If the difference is a stylistic one, it might be a stable feature of the different individuals involved, or it might reflect a more temporary priming effect, whether lexical or semantic or rhetorical.
Or perhaps the observation reflects a genuine difference in the kinds of ideas that the two candidates are presenting, or at least the spin they want to put on these ideas.
Certainly the first solution, statistical fluctuation is a probability, and I don't have the tools to rule it out. I do think stylistic differences contribute to this though. McCain wants his campaign to appear to take a hard line on some issues - first one that comes to mind might be the issue of talking to Iran only with preconditions:
McCain - "We will only talk with Iran with preconditions in place."
Obama - "If the situation warrants it, we may have to talk with Iran without preconditions."
(note that I have no idea if this is exactly what was said, but just providing an example of what I think they might be saying)
But I think the kind of ideas they're presenting bleeds into the stylistic issue. I don't think these two solutions are greatly seperated - if the Obama is the kind of person that will review all of the information time and time again to make a good decision on an issue, he might like to qualify it by using "if" conditions and the like. McCain may be close to the hotheaded candidate that the left is trying to portray him as, and he may be sure enough of himself to make those snap decisions, and is happy to portray himself as overly assertive. After all, quite a few Americans will find that a strength, rather than a weakness, and that's a discomforting thought.