It depends on what you seriously want to do. Is it 600 yds. and under, or 600+ yds. A .223 with a fast twist, geared towards high BC bullets such as 69+ gr. will outshoot a .204 at extended ranges due to less wind drift. The heaviest bullet for the .204 in a factory loading is the 45 gr. SP. The .223 will have much more drop than the .204, but much less drift at extended ranges due to a high BC. Drop is relatively easy to predict, but wind drift is less static due to the unpredictability of the wind over the course of the bullet. At least under hunting conditions (no wind flags). The .204 has a flatter trajectory and more "splat" efect though.
Good luck with your choice... On second thought, why not buy both lol