I'm pretty much as ignorant as Mark.
The answer kind of depends on a couple things. Mostly, what you want (which seems like the M3's). Moreover, if it is your long-term intention to replace the M3 with QS8's surrounds at some point, I might lean in that direction since the "surplus" pair of M3's would make a fine full-range system in some other room. They are probably better by themselves than the M2 (i.e.without a sub).
However, Mark's right about the consensus that the M2 is a better tonal match for the M60. If you are going to listen to a lot of multi-channel music (SACD/DVDA), that might matter. Likewise, if you already have "placement problems", my feeling is that the M2 is going to "seem" a lot smaller than an M3. Also, if you intend to go to 7.1 some day, adding the QS8's and moving the M2's to the rear position might make more economic and spatial sense than the M3's. However, I don't really think you can go wrong in either case.
Personally, I'd get the M2's and use the extra coin to fracture my budget into an EP500.
