Square composition and Rectangle compositions work differently. It is hard to get much side to side or top to bottom motion in the square composition. A square is much more center oriented. I never center anything in a Rectangle.
Personally I can't help but compose full frame with whatever camera. So when I get tired of the square with the Rolleiflex, I switch to the Pentax 67 and go with the rectangle. I have cropped the 67 down to a square far more than cropping the 66 down to a rectangle. When cropping significant parts of my negative I feel like I failed to see what was going on in the camera.