Debating between subbing with full fat greek yogurt or sour cream. I do not want to make my own. Which would you say is a better substitution?
I'd use full fat Greek yogurt because it has more body than sour cream and in that way better replicates creme fraiche. It's not as rich as creme fraiche but you won't suffer too much.
I agree with Amanda I have done it and it was fine only not as rich but nobody noticed
I have mixed sour cream with a bit of heavy cream as substitute. Tastes milder/"fresher" than plain yogurt.
I whip 2 parts heavy cream with 1 part creamed cheese. Seems to work.
Mexican Crema is your best bet, but of the two you mention, sour cream. There is no hiding the assertive (and to many repulsive) flavor of yogurt.