If you truly listen primarily to 5.1 channel music (not HT), and can afford the space and cost, in theory identical speakers are better. BTW that means your material would mainly be multichannel SACD, DVD-A, Digital Dolby & DTS 5.1 music.
However most people want a mix of HT and music, plus don't have a huge collection of discrete multichannel material. Also space, aesthetics and cost make QS8s better than matched M60s for surrounds.
Re soundstage of M60s/VP150 to be less convincing than 2-channel (I assume you also have QS8s in a 5.1 config), if you don't have discrete multichannel material or Dolby PLII or HK Logic 7 to reconstruct surround, you haven't yet had a valid stereo vs surround comparison.
The "surround" modes (not PLII or L7) of many receivers can sound fake and strange, and I see how stereo would sound better. In my experience using PLII to reconstruct surround from stereo is usually natural sounding and generally (but not always) contributes to the experience.
If you have a regular DVD player the best way to evaluate surround vs stereo is get a quality DVD-A music disc with a Digital Dolby 5.1 soundtrack (assuming your receiver does Digital Dolby 5.1 decoding). That way you don't have to buy a special player.
Note there are lots of poorly-mixed surround releases. There are also quality surround relases that just don't use much surround -- that's the decision of the recording engineer and artist. Popular DVD-A albums that makes good use of surround are The Eagles Hotel California, and Fleetwood Mac Rumours.
