Biggest differences between AKC and APDT are that you can take food into the ring in APDT and feed at certain points, but that it is 3 points for every second command. In novice, the signs are pretty similar, and now AKC is adding a bunch of new signs so the difference will be less, but level 3 APDT has harder signs. Drop on recall, directed jumping, baby signals, retrieve.
From a more subjective standpoint, I find APDT judging to be more consistent and the courses to be harder and more interesting.