I've used them both.
I've got the 3.5-15 X 56 on my main coyote hunting
rifle..........works wonderful at 500 and 600 yds.
I think it's the best combination of power and functionability for anything under 700 yds, and I know that alot of guys are using 15X for stuff way longer than 700. I just prefer a little more power for the 800-1000 yd stuff myself.
The good thing about the 15X NF is that you don't Have to
have the power on 22 for the reticle to work as advertised (moa or mill)......15 is good in just about any conditions, even usable in high mirage, which is where 22X can cause problems. very low light.........15 is more usable too.