The R3 has a better barrel mounting system that makes it, in my opinion, a more accurate gun. It does away with the breech end barrel O ring so, that one can’t blow out on the R3. The R3s come with a very nice Walnut stock, the R2.5s were painted black. The trigger mounting has also been improved on the R3.
The early R2.5s had a stack of cones in the moderator, the R3 uses a one piece baffle that makes the gun a little quieter.
The only way that you will be able to get a R2.5 is to buy a used one, I don’t think that Tony has any new R2.5s left over.
Have Edguns, Will Travel.