haven't sold on stockx - I feel like they're having issues with buyer/seller reliability since stockx apparent now charges $25 if you flake on either side (buy or sell). GOAT seems to police a little better, on top of the fact they vet sellers before theyr'e allowed to post.
GOAT is pretty fast - if you have a relatively sought after sneaker at the lowest price things definitely move along quickly. For example If you sell something on Monday, ship on tuesday, and they receive thursday, they usually cash you out the same day. then you request a cashout and that generally takes 1-2 days.
Used to sell heavily through FCNY but got tired of lugging sneakers back and forth then sitting on them forever if they dont sell. With GOAT I just print out the label stick it in a box and drop it off in the mailbox at my office.