I think it is a personal choice. Your not going to see much of a difference between a 150mm on a Mamiya 7II or a Hasselblad. The magnification will be the same, so it is a mater of wether you like square vs rectangular. If it were me, I would use the camera system that you have, buy the 150mm lens, and if you want a square photo, crop the excess rectangular part off when you print.