Bottom line up front, IMO, you will get better quality with Nightforce. That being said, that does not at all mean Vortex is sub-par, especially at their price point. I have experience with other Vortex optics but not the AMG specifically. I am pleased, however, with my NX8. There isn't much more to expound upon with the NX8, I would say it is everything you would expect from NF.
As for reticles, I quite like the vortex EBR-2C reticle but another benefit of Nightforce (at least for the ATACR line, not sure if it applies to the NX8 series) is that when you place your order you can request any of the Horus Vision reticles for a small upcharge.
edit - I should clarify that my NX8 is the 1-8x. There is certainly a difference in quality between the NX8 and ATACR line. For all higher powered optics above 8x I revert to the ATACR line. I have a ATACR 1-8 as well, granted the ATACR is a 34mm tube vs the NX8s 30mm allowing more light transmission, the difference in quality is present.