I've had several aimpoints and acogs specifically the ta31f. Now I'm using a TR21R. It's almost the best of both worlds, of course opinions will vary. It took me a while to switch over in order to gain as much information as I could regarding the accupoint and the mounting system, but it was worth it. It's a remarkable scope priced reasonably. As most have pointed out, the scope allows you to use it at closer distances and at further distances as well. If you are "only" going to use the scope for long shots, then get the TR20. For me, I like the flexibility of the TR21R and the 4x will still get you hits at a 12 inch plate at 400 yds and out with the right dope. So the choice was easy for me because I knew the power range I wanted to stay at. Moving up to the 10 power range where you may want to do some holdovers, ranging, or the like, I choose the Nightforce 2.5-10. Reticle choice along with excellent glass and construction, field of view, positive click adjustments, and value for money was the driving point. So define you're realistic use and pick the scope that best fits your parameters.
As for the mounting of the TR21R, Larue has the SPR-E for this, but I did not use it. Instead I used the PRI Accumount which MSTN had made, according to Paul, specifically for this scope in order to address a few minor improvements. If you want more detail, give Paul a call and he will answer all your questions. Don't get me wrong, the Larue mounts are top notch. I'm using 2 Larue SPR mounts for the nightforce 2.5-10 and for a leupold 4.5-14 scope.