There is not a huge difference between M2 and M3 sound, although for a room that size I would definitely lean towards M3s unless your listening position is quite close to the speakers.

I was able to borrow a pair of M3s for direct comparison with M2s and M60s in a 13x18 room. Here are my notes :

http://www.axiomaudio.com/boards/showflat.php?Cat=&Board=ST&Number=86248&page=&view=&sb=&o=&vc=1

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8