From what I've seen in the hobby, I think believe this about SGC/PSA/Beckett:
- In general, the actual grading of cards is about the same across these three companies. There are examples of each one of them being overly harsh on a specific card, just as there are examples of each one of them being overly generous, and there are also examples of silly mislabeling and other mistakes in all three companies.
- All three companies are unable to catch all alterations that pass through their hands.
- It has been strongly suggested that some submitters receive preferential treatment (most notably some of the work that BODA has done).
- PSA cards, in general, sell for more than the competitors.
Based on this, there are two things that make the companies different. The first is the slab/gasket/flip, which is an aesthetic thing for collectors. I like SGC's the best.
The second is PSA's registry. And the conclusion I've come to is that I have very little need to pay more for a card in a PSA slab just because of the registry. Considering everything I noted above, the biggest (perhaps only) reason PSA cards sell for more is because of the competition the registry creates. When you bid on a card in a PSA flip, you bid against registry people and the privilege of adding those points to your registry. Any individual card in a PSA slab might be better or worse than another example of the same card in a SGC or Beckett slab at the same grade, but the price will be driven higher due to the registry.
That doesn't make much sense to me, as I have no interest in participating in the registry. If I'm going to buy a slabbed card, I'd rather get an example that appeals to me in an SGC or Beckett slab than pay more for an equivalent in a PSA slab (with noting that if an example in a PSA slab is around the same price point, I would consider it).