I've owned both and the latest 2017 vented screen version is a slightly better bike IMHO
Interesting as the adjustable vented screen was introduced in 2015 and that o paper the 15/16 & 17 year models are identical spec on paper other than colo(u)r
Wonder if your 15 was actually an unregistered 14 model?
Anyways presuming the 15 has the following features (highlighted the easiest to spot) I would get that
Kawasaki made numerous minor changes in 2015 that include a revised first gear ratio, different steering stem seal for lighter steering at low speed, stiffer rear suspension for enhanced carrying capacity,
an adjustable vent in the windshield, revised ABS programming to reduce the linked braking effect, a new rear luggage base,
silver bezels on the analog dash meters, elongated mid-pipe heat shield,
change of warning lamp color from red to yellow, tank pad, cushions on passenger pegs, and a more deeply sculpted seat. The revised parts should all be backwards compatible with previous model years. Kawasaki also shifted the North American models to use the oxygen sensor that had previously only been used in the other world markets. Color options of Candy Lime Green for USA/Canada, Metallic Spark Black for all markets, and Metallic Moondust Gray for Europe.