Nick is correct with regard to the bass content in the surround channels. I discovered this for myself when I moved to QS10s for 5.1. I was also very surprised with how the quality and quantity of bass changed when I used 500s with the front A-LFR channels and routed just the .1 and below 40Hz from the surrounds to my 800.

So Nick may very well be justified in saying the M3OW may be better than M2OW for bass but justified only by a 10Hz extension to 70Hz instead of 80Hz for the M2. Does +/- 5Hz around 75Hz really matter?

Also consider this. I have the M3 bookshelves, M2 bookshelves and M2OW. The M2 bookshelves and OW are more accurate, mids and highs are more dispersive and over-all, the M2 is cleaner-sounding without being surgical. The M2s with a sub sound like M5s. I like the bass from M2s better than the bass from M3s and the mid-range is excellent and simply cannot be compared to the M3 mid-range. I can relax to both M2s and M3s but I feel I am really missing stuff with the M3s.

If I was doing Atmos, I would go with M2s for height and M5HPOW for surrounds. The only reason I wouldn't do M5s for height is because I'd be concerned about proper driver integration from the 3 drivers.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated