Customs B - usually has 2 agents working, no lines, and you don't have to re-clear security. Your bags should be checked all the way through.

One proviso - your connecting flight needs to be departing within 6 hours or else they will make you go through Customs A (main customs) and re-enter through International departures.

Made no sense when they did it to me (connecting flight was 8 hours) - I went out through main customs, straight up to international departures, security just said "are you sure you want to go in now? your flight isn't for a while". .. I said "yup", and they let me through (with the usual screening).