I think with prewar cards, there is little differential between realized prices between SGC and PSA although I would give a very slight edge to bigger, popular sets where the PSA Registry may have an effect. For higher grade, vintage cards, there is more of a difference. For example, in a recent Goodwin auction, a 1961 Wilt Chamberlain in SGC 9 recently sold for ~$5000 here:
Link. In the Memory Lane auction that just closed, a PSA 9 Chamberlain fetched over $8000 here
Link. From the SGC boards, the only recent change that I saw is that Michael Goldberg has left the company again due to what is believed an internal dispute:
Link.
I also just want to say that I'm not an SGC hater at all. I use PSA, SGC, BVG, and even GAI (for wrappers). I just don't like the anti-PSA bias on the board, so I try to post a little bit on the other side to show that no TPG is perfect. There is no question PSA does some screwy things, which annoy the heck out of me, and SGC has to exist as a strong alternative to PSA, otherwise PSA will get worse. Right now, my current peeve with PSA is that you cannot get a somewhat fragile card slabbed by PSA. SGC is much, much better in this regard. PSA, if you are reading this, if the card is fragile, just allow it to be slabbed inside a thin sleeve like SGC does sometimes and BVG always does. My biggest peeve with PSA is of course, that they do not allow different card sizes / crossovers to be mixed into a single submission / shipment to saving on shipping fees. The excuse is that they get too many submissions where it's too complicated to do this, but this is either a money grab or pure laziness, I don't know which. Obviously, SGC and Beckett allow this.