I have the M3 and an M6, but no M2. The M3's viewfinder is better for 50mm than anything else, the M6 included. As pointed out by numerous posters above, the M3 lacks 35mm and 28mm viewfinder lines. I find the M6's 28mm lines almost unusable, because they are so far toward the outer edges, and I use Leica's 21mm-28mm adjustable viewer for 28mm and 21mm lenses. I also use a Voitlander 35mm viewer with my M3, and I shoot 35mm a lot, probably as much as I shoot 50mm. I don't find the external viewfinders to be much of a problem. I guess I've shot enough with my IIIc not to mind switching back and forth for rangefinder and viewfinder. If I had to get rid of one of my M bodies, it would be the M6.
Others here are doubtless more knowledgable than I, but I have also read that the M3's viewfinder is much better, but mechanically more complicated than the cheaper-to-manufacture mechanism that Leica began using with the M2 and has continued to use to date. It also has a .92 magnification vs. the .72 magnification in the M2 and most successors; thus, is more precise in focusing 50mm and longer focal lenths because of the higher effective rangefinder distance. I am sure that the M2 is no slouch either, so it probably comes down to whether having the 35mm framelines is important to you.
Regarding cleanup of the viewfinder in the M3, I saw a recommendation for Sherry Krauter, which I can second. I bought my first M3 from her, which she had overhauled, and she does excellent work.