Re: 15x vs 20x +
I really like my5-22 Nxs npr1, I did not like the3-15 npr1 because the reticle was way to fine at low power for lowlight or brush hunting ( reticle was the SFP npr1 in both scopes, I would imagine the ffp would make it even worse.)
You would probably be okay with mil dot since it is fairly bold, but I would stay away from npr1 any lowlight or hunting situations ( this applies specifically to the 3-15)
*I have not looked through the f1 3-15 ffp with the new npr1 ffp reticle, it might be worth a look.