Im actually in the exact same boat as OP. I have Norton 360 and MBAM (Malwarebytes), but my faith in Norton has declined in the past year. I need something new and have been looking at Microsoft Security Essentials. Everyone here raves about its low use of resources (and that its free, lol).. but none of that has me dead set on it just yet.
Because MSE is an all-in-one antivirus/antimalware package, MBAM won't be needed anymore which is kind of too bad, I really like and prefer MBAM. I wish someone could elaborate on these products, it would really help me and OP out.
edit: I forgot to mention I have MBAM Pro, its been awhile since I've used the free version but I think the only difference between the two is autoupdating, and I've even noticed it block some things in realtime.