Looking only at resale value only without looking at the cost of acquisition confounds me, if you are considering cards that are already graded.
Your margin, which should be considered on a percentage basis, requires both numbers.
If you do not overpay for SGC or even BVG, your ultimate margin should be equivalent.
I have tracked margins in my collection for over 10 years, and when it comes to ROI (return on investment percentage),
PSA=SGC=BVG
I realize that I am considered a heretic here for making this argument, but I have the data to prove it.
My mantra is
Buy the card, but consider the holder to determine the purchase price. I am not a PSA guy, an SGC guy or a BVG guy.
If, on the other hand, you are considering grading previously ungraded cards, PSA is probably the best choice due to the Registry effect, for higher end Registry worthy cards.
If you are buying SGC cards or BVG cards with the intent to crossover to PSA for profit, good luck. That’s not my game.