If you have a sub then either speaker would be fine from a frequency response point of view, so the thing to think about is maximum sound level.
The M3's larger bass/mid driver moves almost twice the air so it can play a few dB louder without running into dynamic range compression (ie you turn up the volume but it doesn't seem to get any louder). Figure that the M3s will give you the equivalent of one more "just a bit louder" twist on the volume control - the difficult question is whether or not you need that last twist.
I did some testing swapping my M2s in to replace M60s in a 20x20 loft with vaulted ceilings opening into pretty much the whole house, with sub. The M2s did fine up to "louder than normal TV" but couldn't move enough air for "hey y'all, listen to *this*" HT levels.
Are you replacing an existing speaker - if so, what are you using now ?
Also, does your receiver/amplifier include the ability to set your speakers to "small", ie blocking most of the low notes from the main speakers ? That will also make a difference in terms of how loudly they will play.
If you're wondering why I'm talking about sound level so much I guess it's in reference to your "filling the room with sound" comment. M2s can do that (wonderfully) when the conditions are right, but M3s are definitely a safer bet for a large-ish room. The specs only talk about a 1dB difference in maximum SPL but my feeling was that the difference was more pronounced than that.
The only chance I had to do direct M2 vs M3 testing was in a largely empty bedroom roughly 13x19 feet, without a sub, listening to a variety of music with the speakers on the long wall. My conclusion was that M3s were the way to go for a room that size and the volume levels I typically listened at.
Edited by bridgman (12/13/09 02:37 PM)