I never had the two side by side, but from experience with them both I would say the T6Xi is much more forgiving than the NX8. The recent NX8 has proven to be noticeably better than early release models optically and has evolved into quite a good option but is still a bit finicky. The T6Xi has a very forgiving eyebox, the scope is just easy to get behind.
Going off pure memory, the NF 2.5-20 may have slightly better contrast over the T6Xi, I found in some situations the T6Xi has a little bit of contrast falloff. I am trying to wrap up my T6Xi 3-18x56 review and compared it to NF ATACR 4-16x42, S&B US 3-20x50 and now a new Delta Stryker 3.5-21x44 with the latter scope being late to the party so it has delayed things a bit.
Is the NX8 worth replacing with a T6Xi? A lot depends on what your needs are, do you often shoot at lowest magnification? The Steiner reticles aren't designed for 3x use and illumination is not as bright as NF so I'd say the NF has the advantage there. Do you often shoot in low light, the 56mm objective of the Steiner is likely the brighter scope in low light situations. Do you need a lightweight scope, the NF is 6 oz lighter vs. T6Xi. I would say T6Xi has "better" turrets vs. NX8 as NX8 turrets I find to be mushy but still usable. T6Xi has locking turrets.
One other minor gripe with T6Xi is the battery, it does not use the CR2032 that every other scope manufacture on the planet uses, why Steiner opted for a different battery is just stupid.