PSA claims 2 or even three if there's a significant diff. Don't know about SGC but I have always found their grading to be very good and I've got to assume that more than one grader looks at each card.
PSA seems tougher on card surface; IMHO that is the biggest diff between the two per grading.
PSA gets slightly better prices on eBAY but I much prefer the SGC holders so my strategy is buy SGC and sell PSA on eBAY
