Personally, I prefer the M2. I really depends on how you shoot. I prefer to use a handheld meter when I need to. I like it that my speed and aperture are set, and my 35mm lens has been prefocused when I bring the camera up to my eye, and all I have to do is fire the shutter. You can do this with the M6, but the led lights distract me. So do the framelines. And the patch flare.
The M2's brass wears better than the zinc of the m6. In my samples, which have both been CLA'd, the winding, shutter and shutter release of the M2 are also smoother.
If you mostly use 50mm, the M3 is hard to beat. If not, then the M3 is the "must" for a second body.
If you absolutely need a meter, then the M6 is obviously the answer. If not, then I would get a good user M2 and put the difference towards the glass.