Rather, first of all. Instead of going first to the Macedonians, as he afterward decided. See 1Co 16:5.
Second benefit (δευτέραν χάριν)
Benefit is, literally, grace. Not a mere pleasurable experience through Paul's visit, but a divine bestowal of grace. Compare Rom 1:11. Second refers to his original plan to visit Corinth twice, on his way to Macedonia and on his return.