Hmm. If you want to load your own, obviously the 5.7 will be your own choice. The 5.7 can also indeed be loaded a little hotter and typically with better bullets.
Though honestly I'd just go with a .223. You can down load a .223 about as much as you want if you take the time to learn how to do it, but you can't upload a 5.7 for a 250yd shot like you can a .223.