Well I haven't bought it, only because I have been using SAS before MBAM came out and I bought the Pro version (not having two resident anti-spyware/malware applications installed). So in that regard it is the same simple, because it does things that avast doesn't do when you initiate an avast scan it doesn't specifically scan the registry.
They are also specialist applications much like avast is a specialist anti-virus application; I use a multi-application, multi-level approach to system security. As good as avast is, no single application will give 100% protection. They also offer different options not in avast, some of the ones in SAS Repair tab are particularly useful, see image. Though for the most part these additional options are available in the free product with few exceptions such as resident protection.
I bought mine because I felt it was a good program and since it is a one off subscription rather than an annual one I bought it.