Romney loses, but I imagine he wouldn't have been stupid enough to pick Sarah Palin as VP. He probably opts for Pawlenty or Giuliani. He also would have presented a stronger challenge to Obama in the debates. My guess at the Map is this:
311-227 Obama. The stronger performance probably allows the republicans to pull through in a few more house races, and most likely would have allowed Norm Coleman to survive in Minnesota, which means senate dems would be just short of 60 votes even after Specter's switch, so ObamaCare most likely never passes the Senate.
As for McCain, it's hard to tell. It's possible that he could bungle 2012 and allow Obama to win re-election by something like this:
375-163 Obama
But it's also possible that McCain could successfully appeal to the minorities that voted Bush '04/Obama '08, and have appeal in the upper midwest that Romney never had (Remember that Michigan was considered competitive until the last couple months of the 2008 cycle), and pull off a win:
291-247 McCain
