My head is scrambled but I’ve got my choice down to these two optics. The rifle is a Q Fix and will be used primarily for target shooting and hunting—illumination is a must. For those who have used both—I’ve owned a non-illuminated TMR for several years—what would you buy? My max budget is $2500 for this scope.

There is an MK5 in EE. Not sure if it’s a good deal just sharing.

The ATACR is going to have better optical quality.  The glass and coatings are higher quality than the Mark5.  The Mark5 is on par with the Vortex Razor, but the ATACR is above that level with glass quality competitive with S&B and similar.  To the average person, this is a minor difference because most are not going to glean any advantage until they start working at the extremes.  For people with experience with higher quality glass, they’re going to see that difference more readily.
If I were going to pick one based on nothing else but face value between the two, I’d take the NightForce due to it being better glass.  The Mark5 I think has slightly better features overall, given that it has a wider magnification range, it’s lighter, and the objective lens is 2mm wider.  Minor differences.  The problem that I have is that Leupold charges more money for illumination, and I think it makes them lose competitiveness with their competition like Vortex. I also think that Leupold is trailing in the reticle department because they’re only now starting to put out their own reticles to compete with the overpriced Horus licensing premiums.  With the NF, it comes down to the Mil-XT reticle being 98% of what the Tremor3 is, for less money.  And illuminated.  The better glass I think is the deciding factor for me above all else.   I don’t like the 4x erector of the previous ATACRs, and them now having the 4-20x50 makes a big difference for me in that regard over 16x top end.

That being said, due to my pro discount with Leupold, I would opt for the Leupold given what I can get it for.  NF has decent LE discounts, but they’re not enough that I would opt for them at that level.  Once I get into the higher magnification stuff like a 5-25x and 7-35x, at that point I’m going to opt for the better glass because it’s needed at higher magnification.  That said, I’ve used a Mark5 5-25x and it was still an exceptional optic.  I just think that the discounted price of the Mark5 brings is more in line with what it’s really worth, which is on par with the Razors.  If the normal price was on par with the Razor, I’d say that the Mark5 is the better option.  The pricing is why I give the nod to the ATACR, because the price for them is more commensurate with the quality IMO.

What cartridge, and how far are you shooting?

Wound up with a Nightforce ATACR for the Fix. Thanks for all the advice!

It's a tremendous scope.