Performance. Performance Performance and most certainly not the ratings you see internet writings of paid companies and journalists.
Removal of AVG has fixed problems on more than one computer.
The only anti-malware solution that has no problems with Win 7 is MSE.
I'm going to once again put in this link: Understanding Microsoft Anti-Malware Software 2012 ~ Security Garden
I will not go further. I suggest reading of that blog article by Corrine. You might also find interesting reading the actual and real testing done by some of the "evaluators". In a real world environment, then the only av which performs best and is updated often three times a day is MSE.
This is my last word on this topic.