I think either material would be fine since your coating will provide UV protection, although the polyester may hold up better long term due to its greater UV tolerance. You could always tint the coating or use something opaque to give additional UV protection. The nylon could potentially slacken when cold and damp, but I doubt that it would be anything more than an aesthetic concern.

I have not worked with Dyson's 7 oz. nylon, but his 8 oz. polyester has a rather loose weave and this may present some problems trying to attach it to your roof frame; pulling on it can easily distort or open up the weave. For best results, the polyester should likely be applied relatively loosely (i.e. smooth, but not so tightly that the weave is distorted or has holes pulled open where it is stapled/attached) and heat shrunk to get the final tension before coating.

: Long story about why and how, but I'm building a chicken coop/run and want to
: put a SOF roof on it. I just got the list from Dyson and noticed the 7oz
: nylon and the 8oz polyester are the same price. This is something that
: will be in the sun forever and wonder if the poly will be better because
: of the UV. It will be covered with exterior poly, but still wonder if the
: poly would be safer. The only real stress it will have is during the
: winter and snow cover.