Numismatists will debate this until they're blue in the face, but generally PCGS and NGC are equally accepted in the marketplace. It's akin to the Canon vs. Nikon debate among photographers. Usually people just like one over the other, or they have too much invested with one brand to make switching cost-effective (think camera lenses or registry sets). World coins are where they start to separate. In my experience, NGC's attributions are more accurate, but sometimes still egregiously wrong. One coin came back without a single piece of information (Country, date, or denomination) correct. They also differ in the exact types of coins they certify. If you collect some esoteric things, there might only be only one grading service that certifies that type.