The case is the same for me with my Bronica and any other 6x4.5 system. As for choosing between those two systems, I looked at this link here for a comparison and it looks like the Mamiya gives you more options. I guess the Polaroid 600SE is a simplified version made if you just want to shoot Polaroid photos, as the medium format film back is supposedly going for an upwards of ~$300, while the same back is only ~$50 for the Mamiya system. That being, I'm assuming, because the backs are interchangeable with the Mamiya 23 backs. The price is the same for the Polaroid back on a Mamiya Universal (~$50).