Your seating presents a real challenge for rear surrounds; can you move the sofa out a bit?
I'd also recommend in-ceilings in your case. I recently installed four in-ceilings for my side and rear surrounds. In my case, I have plenty of room behind my sofa for in-ceilings (they're about 12 feet behind), but on-walls were just about impossible due to the floor plan. I'm very happy with the results.