Personally, I like to read through some AV testing results before making a decision. Obviously, personal experience means a lot. But I can say this much. MSFT Security Essentials is a decent product, but it has not fared well in AV tests against other similar products. Neither has AVG done that well, for that matter. See AV Comparatives Dot Org.
Out of the free products, I beleive Avast was the only one to win an Advanced Rating from that group. I can tell you from experience that Avast has a lot more features and from my own experience (as I download malware and test it all the time ( in virtual machines of course) ) Avast offers better protection.
But then again, I have not used MSFT Security Essentials for a few months. I tested it when it first came out. Thought it was alright, but was not that impressed. It may be a significantly better product now. But hell, since so many are free. Just download em all and test em out