2016 had the plunger bump stops in the rails. Many have broken these rails.
2017 Gets new rails that are much stronger. Also gets a new look to the side panels, side panels are also stronger. Kit is welded differently also. The 2017 is noticeably more rigid. Also they came out with the "LE" model witch has the QS3-r shocks which are pretty much the best ever put on a snowbike.
2018 is the ARO. Pretty much all new, narrower track, wider ski, new frame, new track, etc. Build quality and fitment is much better than previous years.
I'd recommend a 17 if you don't want to pony up for an ARO.