A bunch of people have talked about McCain's edge in being able to pick a VP after Obama. The reasoning goes, McCain can tailor his VP pick as a response to Obama's. I doubt this is realistic if McCain is in fact going to announce his VP the day after the Dem Convention, and there are at least two advantages in getting to hold their convention first.
If the Dems are smart, they'll study how Republicans are going to push back (i.e. bracket, respond rapidly) to the "glitz and glamour" of their convention:
John McCain and the Republican Party have their own plans for next week in Denver, including a parade of high-profile surrogates, a Web site touting new attack videos, and a tagline for the Democrats' convention: A Mile High and an Inch Deep.
Two dozen staffers of the McCain campaign and the Republican National Committee will head west this weekend to combat the media coverage the nomination of Barack Obama will draw. The slogan for their opponents: Not Ready '08. ...
Unlike the 2004 Democratic convention, when the GOP did little more than arrange a few interviews, this year's activities are an attempt to distract attention from the main attraction.
My guess is the Dems will copy a lot of these moves when the Republicans hold their Convention in St. Paul, Minnesota a week later. But they'll playing catch up unless the wheels are already in motion.
Another advantage of going first is that Obama will likely get a bigger bounce. According to a Gallup Study, the campaign that goes first get a larger boost in the polls. Perhaps this advantage will be mitigated by the fact that both conventions are late and a lot more people have already made up their minds.
