If you ask me, I'd rather have a 4:5 ratio sensor than the popular 3:4.
645 is 3:4 and all bigger sensors in 36-7 X 48-9 mm. sizes. One good thing about this ratio, it fits perfectly magazines pages.
The 4:5 ratio at current sensor sizes would be 39X49mm. If full frame, 44X55.
4:5 ratio equals 6X7, 4X5, etc, film formats. which in turn fit perfectly traditional darkroom paper sizes.
I know, we are in the digital era now, but 4:5 ratio works better (at least for me) for portraits and weddings. The extra space at the sides in verticals can help a lot in positioning bride veils, the couple and the works.
3:4 ratio is more constrained and 2:3 ratio is just plain unfitable for serious wedding work.
I always enjoyed the extra room I had with my Hasselblad 55X55mm negatives doing portrait shots. I would go for a full frame square sensor, but a 44X55 sensor would be cheaper and just as good. Of course only if provided with a revolving back. No re-attach funny thing.
Just my opinion.