I have seating that is three wide
This alone would push me in another direction, other than M/L, but then again I'm mighty anal with respect to the 'sweet spot'.
IMO, there are much better horizontally dispersioning speakers than electrostatics in general.
Now with regards to your close proxemity of side walls this could be an advantage for M/L.
Myself I bought mine first and foremost for music, if I were to design a home theater I doubt very much if would employ M/L.