The 1 in 12 ROT is by far the more popular twist rate here in Australia for the .223 Rem, as most factory ammunition is produced with 55grs or 50 grs projectiles. However if you plan to only shoot 60grs and above projectiles, you should consider a 1 in 8 twist as the faster twist will stabilize a heavier projectile better than the slower 1 in 12.