Here are my opinions.
PSA generally brings higher prices for key registry issues like T206, T205, Cracker Jack, '33/'34 Goudey and post war issues.
SGC is a very close second on those and equal or not better on the other pre-war issues, especially caramels.
The mantra of "buy the card not the holder" is essential.
Good luck on your journey. From your previous pick ups, you're off to a hell of a start.
Wanted to add - cards that show superior eye appeal for their respective grade, centering in particular, are bringing big premiums in both PSA/SGC slabs.