Every room is different.... I can tell you that at 14x21 M2's were the correct choice for us. I do not think that a few more feet would have made me feel that I needed the M3, but........ If i had the budget available I am certain that I would NOT have been unhappy if I had done the M3 in this space..... We would have more bass and a larger sound stage I am sure.... If you have the ability to hear either pair in a room around this size i would highly reccomned. IF we can help in anyway don't hesitate to reach out. Info@ecny.net