Diesel is the way to go.  Especially if you have a diesel rig.  One made to turn at 1800 RPM will be perfect for efficiency and longevity.  I just went through the same whole process and went through all the different options.  Tons of times.  I ended up going with a haus of a diesel generator…a little bigger than I wanted but the life, efficiency and ease of use will go so, so far