Avast, for several reasons:
MSE has fallen short of the high expectations of the security community...for instance: "Security Essentials boasts live kernel behavior monitoring and other features that should help against rootkits—they didn't. It only detected two-thirds of my rootkit samples. Two were still running and one still functioning as a rootkit after attempted removal...(etc)" From PCMag: http://www.pcmag.com/article2/0,2817,2353447,00.asp
In my opinion, Microsoft would be less aware of it's own operating system's shortcomings, either by ignorance or Corporate Policy of denial (something they have always been guilty of), whereas an outside source would have less bias in the matter.
Architecture within the OS that permits exploits in the first place would be inclined to be overlooked in the software meant to protect it.
It's a bit like the current Toyota "gas pedal" flap: the company knew about it, but denied anything was wrong until outside (non-Toyota) sources began discovery & revelation of the facts.
Ford years ago had the same thing happen with the "Pinto" gas tanks.