.223 is good if ranges aren't too great and the wind isn't blowing too hard. The .243 is a lot more powerful and the higher BC of the 6mm bullets means that it'll shoot flatter at a distance and buck the wind better. It'll also have a lot more energy down range if you like to take 400 yard+ shots on coyote. By 300 yards the .223 is really losing most of it's oomph.

As far as using the .223 on deer...well all I can say is that there is a reason why many states require a minimum caliber for deer hunting. The .223 just doesn't have the energy, sectional density (penetration), or diameter to be a good deer round. Just because the round can, under the perfect set of circumstances, kill a deer, it doesn't mean that it's a good cartridge for hunting them. A .22LR will kill a deer too, but no one would argue that it's a good deer cartridge. For deer stick with a .243 minimum. It's up to us as ethical hunters to ensure that we are able to kill the animal as quickly and cleanly as possible, and part of that is selecting a weapon that is capable of doing the job even if the shot is good but less than perfect. The .223 is so underpowered for deer that using it significantly increases the likelyhood of wounding and needlessly loosing an animal if your shot isn't perfect. Why risk it? Such stories of wounded game just make hunters look careless in the eyes of the public, and that's not the type of press we need if we want to continue this great tradition.

Mike