Just another perspective...
I like my SAI 1-6 and I like my NX8. There are things I dislike about both of them.
I hate the "turrets" on the sai 6. The 1x isn't as true as the NX8 the NX8 that I have is flatter, has less edge distortion and has less mag at 1x.
The SAI 6 has some serious flare issues. Look in the direction of a rising sun and forget being able to see anything through the scope. I've never complained about flare before. I've literally never thought about it before. The NX8 controls flare much better (again I don't think about it), while the SAI 6 is literally not usable in some conditions above 2x mag.
I prefer the SAI reticle to the FC-DMX. The RAF feature is genius. It's really easy to see and bracket your target with, but it never obscures your target. I feel like the FC-DMX has a dead spot in the reticle... Try shooting a 6-10" target between 50 and 100 yards at 1x. If using illumination, the reticle will obscure the target, which I hate (the reticle turns into a 12 moa red dot). If above 1.5-2x, this is less of a problem as you don't need illumination and you can see around the horse shoe. The SAI 6 reticle is much easier to use without illumination, while the NX8 is more difficult for me to use without illum. I like that the t bars/posts come in closer on the SAI than on the FC-DMX. I prefer the NX8 at 8x, as I favor the floating dot vs cross hair in the SAI.
The SAI has a wider FOV, and the ocular is larger which makes the viewing experience nicer. However, the NX8 has clearly better resolution. They both have about the same CA (your mileage may vary). Going from memory here... I don't remember much of a difference in color or contrast.
The NX8 is smaller, the SAI feels large on a 10.5-14.5 build, while the size doesn't really matter on a 16" build. On a build with a 9" rail, the SAI is long enough to protrude into flashlight/laser controls.
I hate that on the NX8 I have to choose between a clear/flat/undistorted 1x picture and a razor sharp reticle, I can't have both. This isn't a problem above 1.5x. This problem exists for me on the SAI 6, but it's not nearly as bad, as the problem really shows itself when using illumination.
I wouldn't say one scope is nice and the other is garbage. The NX8 gives better optical performance in all viewing conditions. It has much better turrets (which is important as I swap barrels - LMT). The illumination is fantastic.
The SAI has a fantastic reticle, the turrets are garbage (which I wouldn't care about if I didn't swap barrels), the illumination is mediocre, which doesn't matter as the reticle is easy to use without illum. The stray light/flare issue is a real problem. The FOV is great and the larger ocular provides a nice viewing experience... Even if the optical performance isn't as good as the NX8.
I bought both expecting to sell one... But I can't pick a favorite, so they are both staying. They both work fine. 6x vs 8x doesn't matter to me. The SAI is clearly a better value, especially at used prices. But is it a better optic... I'm not sure about that.