Having owned the 2014 and 2015 and used the 2016 I can identify the following:
2015 went less rocker than 2014 and the walls of the cockpit got scooped out. They did some minor tweaks to the bottom shape and somehow improved the stability of the 2015 over the 2014. Great board.
2016 Ace initially had less rocker than the 2015 in proto-type (which I tried - was slightly heavier), but they went back to the exact rocker profile of the 2015. The cockpit is more open at the rear area, allowing you to hold your foot square to kick turn easily (which does help - I need help kicking turning an Ace
). The walls are further thinned out and the stability is marginally improved again over the 2015.
