For nature photography which one would be better and why?
Mike
The biggest difference between the two are the color profiles. Personally, I like Leaf's profiles better. The fact that Leaf backs are cheaper couldn't hurt either.
That said, P1 backs have additional features such as focus mask (A lot of owners seem to love this), wifi and output of high quality preview to an iDevice etc. Not a dealbreaker for me, but depending on your workflow, they might be.
Both share the same high quality casing and screen.
Lastly, and this MIGHT be a dealbreaker for you, Only Phase backs work with the XF body now. Leaf support is confirmed, but no timeline is available now.
I'd say, demo them both.
