I have owned a Kahles Gen 3 K624i and compared it side by side with a NF 5-25. Preferred the Kahles in the end, but it was already gone. I then got to compare a 5-20US to the NF and wow... It might not have the "color pop" of the Kahles coatings but damn do I love the S&B. I have not had the chance to look through a S&B 5-25, but I can't imagine it being worth the extra size for the shooting I do. After using the US, I am going to a NF 4-16 on my other rifle. I love how the shorter compact optics make the rifle feel, even if its psychological.
As for eye box, it isn't hard to get behind at all when the rifle is set up right. I have not tried positional shooting, but I don't see it being an issue.