Well, the 223REM does not offer any easy way of loading 80-90 grains bullets, it takes either a rework of the chamber throat to be able to set long bullets far enough out to be able to fill the case with enough powder. Case capacity is less, so I will not have the velocity I can achieve with the 224 V.
Simulation of 223 REM and Hodgdon Leverevolution powder with Berger 85.5 grains bullet in GRT, with C.O.L = 2.2835" in a 24" barrel:
Notice bullet seating depth (0.6433")
Same simulation with 224 Valkyrie, same bullet, same C.O.L, but more powder:
Seating depth is 0.4835"
Pressure is about the same.
Velocity is superior in the 224 Valkyrie.
22. Nosler might be a good choise, yes. Not sure if that cartridge will last though.....
Only Norma makes brass for the 22 PPC, if they discontinue that case the rifle have to be rebuilt to another cartridge. I am not willing to bet on 22 PPC.
224 Valkyrie seems to be the way to go, as manufacturers are now offering a lot of AR-15 guns in that caliber, this cartridge will not dissapear in the future in my believe. So many auto-guns (AR-15 style) out there already in this caliber for them to discontinue this cartridge.