Well I have both calibers in M4A1 (5.56X45) and G3A3 (7.62X51).
They are both useful with unique benefits that are partial to given situation. If your combat zone is mostly within 200 yards then there is really no need for a 308 Win. However, if you are fighting off enemy beyond that rand a 223 Rem. could come up short depending on what you are hitting at.
These two calibers are not replacement for each other. I would really retain both, but if I had to keep one it would be a 308 for its multi purpose usability.