For me, it depends on what kind of place your machine fills in your life (and garage).
My machines are 99% toys. I don't use my machine to plow, to fill deer feeders, or to even check cattle. My UTVs are for getting out with my buddies, my wife, and getting dirty. I want my machine to be fun. That's it's only job - to be fun. If it's not fun, it needs to go. I sold my 2014 Teryx4 800 because it was not fun. It was loud and it couldn't turn the 28" Terminator tires in reverse if you put it in a bind. $600 worth of clutches didn't help. So I sold it for the 2015 Ranger 900. I wanted a machine that was quiet and smooth riding.
The 2015 model with the additional 8 hp was appealing to me because more power makes this toy more fun. If I can still haul a couple of big ice chests full of suds in the bed and throw big rooster tails of sand and mud down in the river bottoms, I'm having a good time. That makes the 2015 model more appealing to me. Maybe it even has a few updates here and there over previous models... Or not. LoL.