From my understanding you have one of 4 options.
1. Buy a .223 and shoot only .223 in it.
2. Buy a .223 Wylde which is set up for both .223 and 5.56
3. Buy a 5.56 which will run both .223 and 5.56
4. Buy 2 uppers for whatever you feel like shooting
The difference between #2 and #3 is with the Wylde barrel the rifling starts before it normally would on a 5.56 to try and get more accuracy with both the .223 and 5.56. Whereas if you buy a 5.56 the .223 may not be as accurate.
The reason for the difference is a matter of pressure, the 5.56 has more pressure than the .223 so if you fire 5.56 in a .223 it can cause premature wear.
This is my understanding of the subject I am sure that others will chime in with more detail and probably correct me where I was wrong.