....to use Avast!, of course But comparing the two? MSE would likely be my fallback if something went wrong with Avast!
Pros: Avast! free has more features than MSE, chief among them is its WebShield, which is a simple local proxy that redirects port 80 (web browser) internet traffic to a different port (12080) to scan anything before it gets to your system. Also very good support forum.
Both are very light on resources, which one is lighter literally changes from day-to-day, depending a virus database updates and how you use your system. Very close there.
MSE checks servers for constantly updated virus database, meaning less local updates needed, and more up-to-the-minute definitions (a big plus). Written by MS, for MS, it has good company support and quicker response to problems.
Cons: Avast! has lackluster company support, typically pushes only 2 updates per day and its behavior shield is still a software fetus, needs work.
MSE's "big plus" of checking servers for definitions could also be a "big minus", if your bandwidth is low, also less shields/layers of protection than Avast!