I 've just chanced on this older thread, and since I have two M cameras and the FM3A, I 'll just add my thoughts. The point I want to make is quite general and it applies equally well to any rangefinder/SLR combo, I should think.
The FM3A is an absolute joy to use, the controls are in many ways very similar to those on Leica. The body is as sturdy and camera deserves its cult following if not for any other reason but for the hybrid shutter alone. The strength of the system is the array of longer lenses (and macro), exactly where rangefinders seem to have a weakness. Both the 105mm f2.5 and the 85mm f2 Nikkors that I use are superb lenses. The normals and the wider angle lenses are not bad at all (although I prefer what's on offer from Leica). But the FM3A cannot replace the Leica when it comes to available light and discreetness. The usual problems that lead some of us to Leica and all the othere rangefinders are there with the FM3A. The mirror slaps like a hammer, it causes vibration and so you need to use faster speeds exactly when you need the most the slower speeds. On the other hand, the 1/4000 s top shutter speed of the FM3A is fantastic if you want to shoot at wide apertures during the day. For these reasons, I find myself taking the FM3A during the day, loaded with some slide film; and the Leicas, loaded with some B&W film, when I expect to be indoors or with little light.
To summarize the point, the two systems have complementary strengths: low light and wide angle and normal lenses for Leica; daylight, longer lenses and macro for the SLR. I do more of the former than the latter so if I had to keep one of the two systems I would stick with my rangefinder.