Choose the QS2 if you have a smaller room and go for the QS4 if you have a larger listening area.
Don't be too concerned about the tonal match b/w the M3 and VP150. Although the M2 could be listed as being a better timbre match, the difference will not be so great that you would hate the sound, at least i doubt it.
Many people have mixed brand systems and in this case, you are just mixing models which are still very similar in design.

"Those who preach the myths of audio are ignorant of truth."