One down side of Romney as President is that he seems to have no principles at all, simply saying whatever he needs to say to get elected. Another is that because he is unprincipled, he is likely to be controlled by the noisiest or most powerful voices in his party. And as a very liberal person, I cannot see anything positive in a Romney Presidency.
The down side of Obama's reelection is primarily one pointed out in an earlier response, that if Congress is in the hands of the other party, he can accomplish little or nothing. If he has a Democratic-controlled Congress, I think he can effectuate changes that will be good for all of us, including a stronger stimulus program, which we desperately need.
If you look at the Constitution, though, you will note that the executive branch has very little power on its own, and so, whatever the pros and cons of either as president might be, presidents in general cannot do nearly as much as people believe they can.