You are technically supposed to use the Visitor line since you do not officially have permanent residency yet.
But last year, me and my wife returned and used the "GC/Citizens" line. I had my green card but she was on AP. After taking care of me, the officer did something with my wife's passport (stamped?? i dont remember) and then asked us to go to the room where they processed the APs. I was afraid the officer would ask my wife to go back to the visitor line, but she did not.....maybe because I had the green card.