It depends partly on which terminal you're at - the Economy buses sometimes fill up before they get to the 6/7 (and maybe earlier) and you can miss a bus or two because they're full and LAWA isn't organized enough to figure out that they need a significantly higher bus frequency at peak times/holidays and that if they're filling rapidly in the north complex they need to send empty buses straight to the south complex. I've usually had pretty good luck at Economy, but have also had some long waits. If you don't mind a short walk across the street, you should also be able to hop on a City Bus Center shuttle if it comes before the Economy shuttle. P6/7 are better for driving out of the CTA than P3/P4. I don't fly out of the north complex much, but East Way and West Way should make P1/P2 comparable to P6/P7 for getting out.