Had the ATACR 4-16 Mil-XT and the Mk5 with illuminated TMR and semi-autos. Despite some overlay on specs and being lumped into the same class, they feel like vastly different optics. It is not lost on me that my reticle choices were different and were I a Horus/Tremor person my input may be different if they were both T3...but this was my feeling:
The Mk5 felt more like an LPVO that just kept giving me magnification. It didn't "feel" like a full-size optic. It was light. The image was good but the FOV felt constricted. IMHO, with a lack of options in the current market, this feels like the ideal choice for a 16-18" SPR or semi auto where weight is a consideration without having to sacrifice a ton on optical performance.
The ATACR on the other hand does not feel as if it gives anything up to a regular full-boat precision optic...both good and bad. Despite the size footprint, I've never found it "light" on a shorty bolt gun or AR15. Mine felt very touchy on parallax and I'm not sure the reticle choices are optimized for the magnification range of this optic if you intend to run it much on the bottom end magnification.
I don't think you can go wrong, but if I had to get the absolute MOST out of my optic in pushing performance/distance, i'd probably lean ATACR between the 2.