The Razor is functionally identical which is fantastic, I have not been able to compare them side by side but expect the ATACR to be just as awesome. Here's how I'd use them for example:
NX8 for an SBR, 16" and under lightweight setup, think of it as an Aimpoint Micro that has magnification IMO, great for a defensive/duty setup.
Vortex Razor Gen II HD-E for a General Purpose Rifle, 3 gun, Defensive/duty setup (and at a great price)
1-8 ATACR same as the Vortex with additional capabilities at medium to longer range distance with FFP 8x and the grid type reticle. Also very well suited to larger than 5.56 caliber semi-autos.