Like others mentioned, 2017 is improved as expected and I would appreciate torque vectoring, slightly quicker steering and LED lights. Obviously, this is very personal preference bit I don't care about sound isolation in Subaru and different infotainment system.
But there are 2 important things that I like more in 2015 XT.
For me, 2015 XT has better Si Drive programing, I and S mode are full CVT and S# is stepped but with the same ration spread as I/S in manual mode, 1st and 6th/8th "gear" are identical. So if you drive in S#, you have all the benefits of more aggressive torque curve (and gas pedal tip in) but you can still cruse in 8th "gear" and be relatively efficient. Only I mode in D is slightly taller than 8th in S#, but not by much.
For 2017 XT, Si Drive lost full CVT option (both I and S modes are stepped after certain percentage of gas pedal) and S# has 8th steps that are much shorter (but 1st is still the same) and 8th gear is more like 5th in I/S mode.
Therefore, when cruising at constant speed, S# makes no sense.
Second thing is very personal but while 2015 XT is not easy on eyes inside/out, it still looks slightly better to me that later models with it's slightly simpler wheels (still ugly) and less fake chrome inside (on vents for example in 2017 that has reflection on side windows). I know I am minority but I also like more simple look of 2015 non-NAV head unit vs more modern 2017.