There are basically two principles involved: 1. a lot of positive press is a good thing, 2. though veeps don't actually help much, they can screw things up pretty badly. Veeps do not bring their home states, etc.
The Dems go first so the GOP pick will be a response to the Dem pick. The Dems have five sorts of choices -- Clinton, Gore, Biden/Nunn, Bayh, and Kaine/Sebelius. I would rank them in that order. The higher-profile the veep the better; it'll focus attention on the Democrats but not on Obama, with whom the public is a little bored. I used to think Clinton would be a bad idea and to some extent I still do, but everyone else I'd have liked (Webb, Edwards, Rendell, Strickland) is out, and the remaining "sensible" options are impossibly boring. Gore would bring a lot of good feelings but might be unsound on the offshore drilling issue, which Obama wants to compromise on. If neither of those is possible, it seems like Biden is the best of the rest of the pack, being an established and articulate figure that a lot of people have kind of heard of. Nunn is a paler version of Biden. The others, besides being anticlimactic and killing the news cycle, will not attract attention and have no electoral upside, being mediocre speakers and having no real story to keep the newspapers busy. (cf. Jim Webb) There is always the danger that Clinton will be a disaster, but Republican attacks on Clinton will energize the (overhyped) PUMA types.
The Republicans have three options: Huckabee, Romney, and nonentity (Pawlenty, Crist, Jindal, etc. -- unknown figures). Romney would be a terrible choice -- 1. he has a turd for a personality, and no one liked him in the primaries; 2. he's a corporate fatcat and will hurt in Ohio. Huckabee appears not to be in the running, which I think is silly because, if Obama plays it safe and his VP announcement sinks in the press, it's a good opportunity for McCain to pick up some momentum by picking a well-known VP. Of the available candidates, it seems like Huckabee, though risky, has the biggest upside -- he's close to the Jesus freaks in Colorado, and to the Appalachian types in Ohio, and would be an effective campaigner in both states. (On the other hand it's possible that enough people are terrified of Huckabee and the religious right that he'd sink the ticket.) If Obama picks someone high-profile, McCain should go with a nonentity because he can't win the celebrity war.
The real question is whether McCain can win on "acceptability." My sense is that he can't, quite, against Obama alone -- Kerry nearly won in 2004, and did about as badly as Obama with the Appalachian types -- but might be able to against Obama/Clinton. The problem is he hasn't got too many options against Obama/Clinton.
PS I'm told that Tom Ridge isn't a nonentity. Who knew.