McCain's choices are simple: attack Pennsylvania, attack Michigan, or defend existing territory.
Prior to the outset of the General Election-proper post-Conventions, the strongest indication we will have of McCain's choice in these respects is his choice of Vice President.
Pennsylvania should be out of reach for McCain but he could chose former PA Governor and ex-Secretary of Homeland Security Tom Ridge. Ridge is a moderate Republican on social issues (including his pro-choice stance on abortion) and national security hawk who is still popular in his home state. Ambinder laid out the strengths and weaknesses of such a choice here. This would be McCain's biggest gamble, risking his existing Republican support to bring over enough Democrats and Independents to turn the Keystone state red.
However, Ridge's abortion position is likely a deal breaker. As such, if McCain is minded to play attack elsewhere, his best alternative is his own once-fierce rival Mitt Romney. The Fix lays out the pro-case here and the anti-case here. Romney would help McCain with fundraising, shore up McCain's support on the Right whilst also playing off his Michigan roots as the son of popular ex-MI Governor George Romney. His slick communications skills and strong business background would be further assets to a candidate weak in both those areas.
Should McCain eschew his best chances to optimise an attacking strategy he would likely go with with Tim Pawlenty, the Republican Governor of Democratic Minnesota. The Fix lays out the case for Pawlenty here and the case against Pawlenty here. Of all his options, McCain has the best personal chemistry with Pawlenty.
My instinct tells me that McCain will go with his own personal comfort choice of Tim Pawlenty. This will have little effect on McCain's chances in Minnesota and thus the choice will reveal a McCain strategy predicated upon defence of existing Bush states. Furthermore, he will defend himself on issues, on communications and on age by having a bright, articulate, young Governor with whom he has a strong rapport at his side constantly. Such a move will be a mistake, but it is a mistake I think McCain is likely to make.
McCain should choose Romney but he will choose Pawlenty.