I am a huge Kubota fan, and run the trails with my RTV 900, but to me, that is where its weaknesses really show up. Most any other SXS out there will out pull you on the hills, but the 'bota really struggles in M, and is super slow in L. Also there is no suspension to speak of. You get into the "woops" and it will absolutely beat you to death. I haven't run a diesel ranger, but was running with a gas ranger. That thing ran circles around the RTV on the trails. It has more ground clearance, and did better in the mud, up hills, and less drag in the sand.
All that said, everything is a compromise. I will not be getting rid of my RTV. It is a work horse. I have tricked mine out to do things it wasn't really designed to do (in my opinion). I have no doubt that Peanut knows what he is talking about, and the welds and build of the RTV are stout. If you are MAINLY looking for a trail running machine, you might explore all your options. If you are looking for a dang stout work machine that is quite capable on the trails (with a few weaknesses), will last forever, and sips fuel, the RTV may be the machine for you.
Just my .02
Doc
Edit to add, looking into a turbo kit to help with the hill climbing.