Romney - 50.1% - 274 EVS
Clinton - 49.9% - 264 EVS
Clinton would probably start in the driver's seat, polling at about 54% with over 300 electoral votes. However, after months of campaign, smear tactics, debates, etc..Romney would close the gap.
I think the campaign would come down to whether or not Clinton had worn out her welcome on the political dance floor. I think she will if she is nominated. People will get sick of her and choose the fresher face.
Having said that, Romney is a flip-flopping mormon. Thus, if he wins...it wont be by a huge margin.