There are no mistakes in buying any set of speakers. In your particular case, the only difficulty you may have is getting the right surround field setup properly to create that 'sweet spot'.
I've found the QSx to be more forgiving in this regard but if you have the space around your listening area to tinker with the speaker placement, you should not have any problems tweaking all the fronts and surrounds to come into a nice harmony.
It is typical that users buy larger main speakers than surrounds but i believe both setups will provide a great surround experience.
