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You can use either.
I would recommend rs785 because you can easily switch over without changing the hose end. They (RS805 flatmount & rs785 postmount) use the same m8 fitting.
r785 has a banjo fitting where the hose angle is adjustable but you dont really need that.
rs785 is a sleeker design with the hose on the inside.
so -> https://www.bikeinn.com/bike/shimano-road-calipers-pos-mount/135996835/p
if the picture is corrector bike24 :) (edit: ok not cheaper ... for once)
Any ideas?
Cheers