Go with the M60s. If you're looking for a comparison, go to Myer-Emco or some other high end audio store and listen to the Bowers and Wilkins 703s. I hear that they are very similar. While it is a soer sopt for many M50 owners, I think that there are more M60 owners on the board than 50 owners (both groups are very happy, mind you).

By the way - I ended up with the M60s, but it was recommended that I get M50s for a 20X12X9 foot room. I wanted to avoid the "what-ifs" of an added driver.