Never owned an ATACR, although I know the scope get a lot of love here. I do own an NX8 2.5-20 and really like it. The NX8 has very good glass, although I suspect the ATACR will be even better. ATACR will probably run you about $500 more, if that price differential is important to you. Personally, I think the larger magnification range makes the NX8 slightly more useful. Depending on your eyesight and the size of the target having an additional 4x on the upper range might be useful. And 2.5x at the lower range will make the gun more useful at fairly close range.
The one thing I would caution you about the NX8 is that it has a very shallow depth of field. With the parallax set for 100 yards, even something at 300 will not be in focus and require an adjustment. I suspect the ATACR will be significantly better in this regard.