I was running MSE as my realtime scanner until the beginning of December, but was getting frustrated at its tendency to scan every file in a directory every time I browsed through my storage disk. I downloaded the demo of WSA at the beginning of December and have been very impressed with its features, to the extent that I upgraded to the WSA Complete package this weekend.
Obviously, with being a cloud based service, WSA wouldn't be as suitable for an offline computer but otherwise I think the theory behind the protection is solid when compared to the traditional definitions based security programs. The client itself is extremely light on system resources (currently 500K of memory as I type this) although the password vault and online storage features of the Complete version are just rebranded Lastpass and SugarSync so if you install those the memory use increases to about 25-30MB.
I haven't bought a paid antivirus suite for the past ten years, but honestly the feature set of WSA has won me over so far. There's also a 70 day no-quibble return policy if I change my mind. If you're extremely security conscious the program also plays nice with other AV, so you can install MSE for added protection if you fancy.
Finally, I got the 1 Year Complete 3-user package from the American Webroot website for 50% off, so works out about £30 when converted from dollars. The offer is available until 31/12 by the looks of it, so if you decide to buy then I recommend that method. Try the demo fro a few days first though. Typed more than I intended to here, so wall of text over.