I'll kick this off. I have M60s, M2s, VP100 and Paradigm Atoms. Have owned Quads, Rogers LS3/5a and KEFs in the past.
Bought the M2s first. Never felt them to be bright (in fact I loved the sound from the moment they came out of the box) but I did find them really edgy at first. That went away after a few days, probably will never know if that was brain adjusting or drivers breaking in.
Bought the M60s after the M2s. Relative to the M2s I found the 60s to be quite mellow... probably because the extended bass made the sound more balanced.
In my bedroom upstairs (very live walls, 13x17, blinds rather than drapes) there might be a bit of ear fatigue. In the rec room downstairs (13x23, heavy drapes, lots of furniture) there's not a hint of fatigue -- if anything I would probably tweak the treble up a bit if I could.
I haven't found the 60s (or the 2s) to introduce sibilance where there is none on other speakers, but on any recording where you hear sibilance on other speakers I guess you might hear a bit more on the M60s. I think the way to think about it is that the M3s probably reduce a bit of sibilance in the recordings, not that the 60s introduce it.
Bottom line; I don't think this is a "good or bad" situation. The M2/M22/M60/M80 family will sound a bit more "clear" than the M3/M40/M50 family, but if there are bad things on the recording the bad things will be more clear just like the good things.
I guess one question is "what would you most like to improve about the M3s ?". If you're looking for great bass, smoother/more open sound at high volumes etc... then M50s will give you all that and more -- if you wish there was a bit more "clarity" sometimes then the M60s will add that but will probably expose a few more warts in your music collection.
My first thought would be that it's probably easier to go with 60s and make them sound like 50s with room treatments than it is to go the other way. The difference is supposed to be fairly subtle, a good example being the M50 vs. M22 comparison :
http://www.axiomaudio.com/boards/showflat.php?Cat=&Board=ST&Number=47919&page=&view=&sb=&o=&vc=1
Also :
http://www.axiomaudio.com/boards/showthreaded.php?Cat=&Board=HT&Number=78117&Search=true&Forum=All_Forums&Words=50%2022%20which&Match=And&Searchpage=0&Limit=25&Old=1year&Main=77752 
