Here-s my experience with MSE (some time ago): Really simple interface, targeted at every Windows user. When it came out it had really good detection rate (maybe it still has) and almost no false-positives. I am sure that the low FP rate is still there because I don't think that Microsoft can afford having many false-positives (companies will complain if their files are detected as malicious). It includes cloud technology: Dynamic Signature Service Feature and SpyNet (scary name). On the downside, removing a file (even if it was not running) was slow. Full-scans were extremely slow and RAM usage was high. Again, this was some time ago.
Avast has good signatures too and it is light on system resources. Not all shields are useful to me, I only install the webshield and the Network Shield. Since I mainly use webmail I have no use for the mail shield and the only p2p that I am rarely using are torrents - there is plenty of time for the AV to scan a file. Avast evolved allot, v5 was practically a new product and it was mostly bug-free. V6 introduces important technologies, but at this point they do not work at their full potential.
Both products can't protect the system on their own. Maybe if you have a low security risk, run under a standard account (LUA) and have a router that includes a firewall an av can be sufficient. You need backups too. But in most cases you'll need to pair it up with some other form of protection.