So after running the ATACR and having had the opportunity to compare it to the Mark 6, I definitely feel the ATACR is the overall better optic and superior in several areas. One item to note up front is that the ATACR, like most LPVOs, is not a true 1x, and I did detect some slight magnification at the low end, so essentially it's more or less a 1.1-8x. Whereas the Mark 6 appears to be more of a true 1x in terms of what you're seeing when looking through the glass. Shocking I know. However, I suspect this would not be a deciding factor for most people, and it is easily overlooked when you consider the other areas where the ATACR shines.
At 1x there is virtually no edge distortion in the ATACR, whereas it is noticeably visible on the Mark 6, and this holds true across the power band. I compared each optic at 6x, and the ATACR appeared to have slightly more magnification (read: slightly less FOV), but the optical clarity was superior to the Mark 6. I was able to make out finer details when viewing objects at a distance, and the image appeared brighter and the colors had more pop. At the low end the ATACR clearly had the larger eyebox, which translated into being more forgiving when set at their respective max power settings. Chromatic aberration was more prominent on the ATACR at 8x, but I've yet to come across any top tier optic that doesn't display this to include the likes of S&B, Kahles, etc. Nevertheless, it's not as sensitive to slight head movements as the Mark 6 at max power. Bottom line, the ATACR proved to be the better optic throughout the power range.
The Mark 6 is known for it's nice DLV illumination (but suffers from "flicker" due it being eye-alignment sensitive and not very forgiving as it relates to consistent head positioning), and the reticle illumination on the ATACR at setting 9 was equal to the Mark 6 at its highest setting (7). The FC-DM was "ludicrous daylight visible" when turned up to max power (10), which didn't wash out even against the brightest daytime backdrop. The ATACR also features additional NV reticle brightness settings, giving it even more of an advantage over the Mark 6. I spent Sunday high up in the Rocky Mountains engaging targets at varying ranges (200-500) and had absolutely no problems making consistent hits on steel plates at 500 yards against a wooded backdrop without having to use any illumination at all.
The elevation and windage adjustments are much more refined on the ATACR, crushing the Mark 6 in this area as well. Very positive click adjustments, similar to an S&B in feel, but more refined audibly, and I harbor no doubts that it tracks true (although I did not test tracking). The CMR-W and FC-DM are pretty much equal in capability in my opinion. I really like both, but a slight edge goes to the ATACR for execution considering the thinner mil hashes and a more refined sectioned cross hair, which does a better job of drawing your eye to the center segmented circle. Weighing in at 6 oz. heavier than the Mark 6, the ATACR feels much more robust, yet does not necessarily feel that much heavier than the Mark 6. The weight difference did not affect the balance of the rifle at all (SR25 ACC). The ATACR gets bonus points for the added PTL and beauty rings if you decide to run it uncapped.
To be perfectly fair, the Mark 6 is still a very capable scope, and the fact that it can still hold it's own against the ATACR is a testament to it's design. However, the ATACR takes it to another level overall, refining the areas where the Mark 6 could use improvement, and I personally feel it's worth the price of admission.