I would vote for a third party candidate in this scenario as a protest vote since there really isn't that much difference between the two tickets. Both tickets consist of extremely middle of the road Senators. It would be like choosing a favorite or least favorite between Justice Scalia and Justice Thomas. They may differ on a few particular issues, but by and large they are virtually indistinguishable.
Actually, you're right to a certain extent. Nelson and Snowe aren't all that far away from each other. Although I consider Snowe to be more economically conservative, with Nelson being more socially conservative...but that's just my perception, don't think it matters to much.
However, for some reason, I find liberal Republicans much....much....much....much more appealing than centrist Democrats. It gives me hope that their party isn't completely hopeless, and overall my views resemble Rockefeller Republicanism more closely than Blue Dog Democrats' ideology.