My decision was based on where I wanted to use my points. SPG has some pros and cons, but the more limited hotel basis did impact me. (For example wanted to use points for a free stay in Spokane in January. SPG NO options) I like to use my points for Europe and once again while Marriott's far from "everywhere" they had better overall options.

However, that was also a factor for "earning" Lots of places I go there's no SPG option. IMHO that's the big drawback with them. If you don't only travel to "big cities" it's harder to utlilize.