As long as you get their new logo, it will be post buy back.
It's holographic vs l.e.d..
Personally ive had a 512, exps 2-2. T1, h1, h2, holosun 515gt.
All have is ups and downs. Even aimpoints and holosun have thermal drift. Eotech claimed it had ABSOLUTE zero thermal drift and on a government contract they found that to be false. I think it was up to 2 or 3 inches at 100 yards. So they got sued. But even after all that crap, the government bought thousands more units to supply the armed forces. I think it was just an easy and cheaper way to upgrade all their old crap. I did the buy back program. At night, eotech was king to me. I love the projected reticle. But during the day I couldnt see shit. It would washout even on semi cloudy days. If I used a light on my ar15 it would washout as well. Especially with light splashback. The battery life was crap compared to l.e.d. but with a projected reticle it was a trade off.
I do wanna try their new green reticle though. Idk if it'll wash out with my eyes in the sun and with a weaponlight.
In the end , its all preference. Expss comes factory with a qd mount while xps has a torx or hex head screw. The 2, 2-2, 3-0 etc refers to what reticle it has. And iirc the 3 series are NV. And green reticles are only offered in the 3 series.
Only exps and xps have side loaded batreries because the 5xx series have problems where overtime the connections will fail due to recoil. Cause the batteries are inline of recoil.