I have an Ares BTR, and here are some thoughts:
For the money its a great scope. glass quality is decent, mines tracks really well, I shoot out to 800 at my range and I can dial between ranges quickly and easily with confidence. After a morning of shooting I did a zero check at 100 yds and I maintained my original point of impact no problem.
Comparing to the FX Id say the glass in the fx is a touch nicer (at least to my eye) Had them side by side, long with a couple sightrons and a night force shv.
Id rank the glass as follows:
1)Nikon FX, it was just clearer and the resolution was much better, better turret though
2) Athlon Ares BTR, not as clear or as much resolution as the Nikon but very close. I prefer this reticle over nikons offerings so I'm happy with my decision. Turret feel lacks as well compared to the Nikon
3)Sightron SIII great scope, parallax adjustment wasn't as forgiving as the nikon or Athlon. and the glass is just a touch behind both
4)Nightforce SHV, I just wasn't impressed with this scope at all, the turret was the best IMO but the clarity was just meh, in Canada this carries a $600 premium over the Athlon and it was at best on par with the rest at full magnification.
At 15x in all scopes they were all very close, my comparison above is at full magnification
One thing I don't like about the Ares BTR is its only x50 and not x56 but I don't shoot a lot of low light with that particular gun so it was a must point for me.