I looked at both PCGS and NGC and chose to go with the latter. I just has a better feeling about them. Overall, I have found their grading to be strict but fair and that has made me much more careful about my purchases.
I think the differential with regard to auction prices mainly applies to US coins and, there, I think that PCGS has a stronger presence amongst dealers which helps push prices. I'm not sure that there is much of a differential for non-US coins. At least I haven't noticed it.
The NGC vs PVGS discussion is one of those 'wars of religion' that never finds a conclusion. In reality, both grading companies are very reliable, quite consistent and extremely professional. They do, however, grade to their own proprietary standards. I followed an online discussion on this subject and a very reputable US dealer commented:
"In my observation and experience, NGC started to award higher grades to coins with original surfaces where the surfaces were devoid of marks, hits, scrapes, etc... whereas PCGS started to award higher grades to coins with great luster or extremely attractive toning. Over time, this started to weight the pools of coins seen in NGC and PCGS holders so that high grade, valuable coins in NGC holders were more likely to have muted surfaces or neutral-to-unattractive toning while coins from this same niche in PCGS holders were more likely to have good luster, cool toning and better arm's length eye appeal. Obviously, this was not universal and did not happen overnight, but I would think that by 1995-1997 there was a clear distinction between the pools of coins graded by the services that were available at auction or on the bourse. In my opinion, the early decisions by PCGS and NGC ended up harming NGC while helping PCGS establish a firm hold on the top spot in the eyes of many folks."