If I could only have one - and not yet owning a 204 or the like - I would pick the .223. I've always had super performance from 26.x gn of N133 under a 50 V-Max moly/205m in an RP case. At 3550 it is rubbing shoulders with a 22-250 and is reliable to 450 yards and on calmer days, I've made a few 500 yard shots. With a 9 twist, 500 to 600 should be quite "doable".
Barrel life is looooong on a .223, it doesn't heat nearly as bad as some faster - more powder rounds, cheap to reload, etc, etc. But, having said that, it becomes limited in the wind, and for long shots beyond 450 yards the 220 Swift steps up to fill the "favorite" role. And, according to my balistics calculator, the 204 Ruger or 20 TAC will pace the 220 Swift (trajectory speaking), with less powder, heat, wear, etc.
On a PD safari I take the 17 Mach IV, a .223, 6BR, and 220 Swift. Each has it's place as PD shooting offers several different styles of shooting and each style tends to favor certain calibers, I find.
I think the .223 has been the PD caliber to which all others are compared for a long time. Now with the 20s coming of age, it remains to be seen whether the .223 will share the top row with the 20s, or have to step down a level.
No doubt about it, it appears the 20s are here to stay!
P.