Over the FM2, the FM3A has aperture priority auto exposure, and slightly better flash integration. It really is more like an FE2, though the flash features are slightly different. You might want to compare costs to getting a good condition used FE2, like from a place such as KEH.com.
I think the FM and FE combination are a good complimentary pair of 35mm SLRs. Any of the versions, from nearly any years, would be good choices, though the FM2 and FE2 would be much newer, and if you look at enough places you might find an FM3A new.