For testing purposes it seems both AVG and AVAST take up more or less the same amount of RAM and it seems lesser than the one taken up by MSE. The new AVAST seems packed with nice options as of late too (sandbox, common software updater, browsers toolbars cleaner). One thing I really dislike about AVAST is the need for registration (it's more a nuisance than anything to me) and too much "get the commercial version" ads everyone (even within the installation process). AVG seems to perform well too but I think it lacks some of the latest AVAST features.
As far as interfaces are concerned, I still prefer MSE. Some would say its inteface to be very "spartan" but very tidy and uncluttered.
For me I like MSE on Vista/7/8 but for WinXP I prefer AVAST or AVG. I don't know why but Microsoft hasn't been able to deliver a satisfying experience for MSE running under XP.