Caliber differences are in weight and bullet diameter, primarily. 7.62x39 typically fires a 123-grain bullet in roughly a .31 caliber, the .223 was designed around a 55-grain bullet (now commonly uses anywhere from 40-80 grain projectiles) in a .22 caliber.
Which is best, full or semi-auto, depends entirely on the anticipated use. If military, the full auto version is preferable. If not military, the semi version is preferable. Neither is inherently "better".
May be irrelevant anyway since full-auto AKs are quite expensive, hard to find in transferable form, and may be prohibited locally in your area.
Denis