Can't go wrong with either.
I have the ATACR on a 308, and the Kahles on a 16" 5.56
- Kahles eyebox 'feels' a bit bigger to me, but they're both great, and have excellent glass.
(I wasn't happy with the image clarity on the NF out of the box at distances beyond 125yds+, as the images weren't crisp/resolved fully, and sent it into CS - not sure if they changed a lens or adjusted the fixed parallax, but it's perfect now and every bit as good as the Kahles image.)
- Kahles is slightly easier to stay in/on when dialed up to 6x, as compared to the ATACR at 8x, but I haven't had issues with either.
- FOV is a bit cleaner in appearance as well, if you don't like a busy tree style reticle, although the NF FC-DM reticle is by no means overly busy or cluttered.
I prefer the ATACR for use as a larger Aimpoint style red dot, as well as for max mag when dialed up for holdovers (I keep the elevation turret capped on both) but that's also taking into consideration that I can stretch the 308 out a bit farther than the 5.56
I think you'll have to boil it down to which options/features you prefer, as well as reticle choice and focal plane. The ATACR reticle, being FFP, changes with magnification, but the reticle stays the same size *in relation to the target* irrespective of magnification - whereas the Kahles reticle, being SFP, always stays the same size, irrespective of magnification, and reticle/holdover values change relative to the magnification setting. I think of the Kahles SM1 as a much more refined Eotech style reticle, with elevation holds and a very fine center dot, which is better for pinpoint aiming vs. the ATACR center dot.
The illumination on the Kahles increases/decreases with the rheostat control, and is always on, as with an Aimpoint or Eotech, whereas the ATACR has an off position in between each brightness setting, which is my preference.
The Kahles lends slightly better to a 5.56 in my opinion, considering the caliber ability and distances with which it will be used - as well as the overall weight and size of both - but you'll be golden with either, and an argument could be made for either of them in regard to a specific rifle platform/caliber. The extra 2x will help with target evaluation/PID etc., but the ATACR center will also be covering more of the target at max mag, as compared to the Kahles at max mag.
Reference this thread about the ATACR, and post #160 from 00bullitt in particular:
https://www.snipershide.com/shootin...r-f1-1-8x24mm-34mm-tube-models.6798633/page-4