I will first state my own bias...I have two ETRSi bodies, both purchased new 20 years ago. I prefer leaf shutter lenses over focal plane shutter, as I did wedding work and portraiture, in addition to my own free time photography. I also prefer both Mamiya and Bronica over the Pentax simply because I dislike the balance of the body and lens...the integrated handle is at the very back of the camera, putting ALL of the weight forward, rather than some of the rear of the camera serving to counterbalance somewhat.
The ETRSi is 220 grams lighter than the SQ-Ai...about 8 oz. lighter