On all of the models you mentioned, I have not seen many good things posted about their gas models, which goes along with my personal experience. I strongly recommend the diesel models if you are using it for work. If it is more for fun, I'd suggest the gas powered Rhino. Rather than just looking, I've owned nearly every model you are considering. I've settled on an RTV for "work" and also bought a Rhino for light work and fun. I'm well pleased with them
I've heard good comments about the diesel HPX Gator and I personally like it as well. I would have purchased it rather than the RTV if it had power steering, hydro transmission (it has belt drive), and a larger bed. The RTV bed is larger and has more capacity with a true hydro dump. Besides being heavier, and heavier built, the RTV's biggest selling points to me was the smooth and powerful hydro transmission and power steering. The belt drive transmissions have proven to be reliable (except when wet), but they are not nearly as smooth as the hydro. I'm sure you'll like whatever you get; good luck!