Regarding 15's post. Most all of the present AR Manufacturers chamber their weapons for 5.56. Some of their literature states 223/5.56 Cal. This is only so they can relate to the caliber. As he stated 5.56 and 223 will shoot fine in the 5.56 chamber. But for example you had an older bolt action rifle that was truly chambered for 223 Remington you would not want to shoot 5.56 in it. It would be dangerous to do so. The 5.56 has a higher chamber pressure and different chambers. I would advise to choose a 5.56 AR over the 7.62X39when you get one. Later if you wanted a 7.62 X39 you could just buy a complete upper and magazine and you would be good to go!
Also delivery times are starting to get longer and longer due to the present demand for ARs. So you probably should get it as soon as possible.