I went through the same dilemma, I knew I wanted a diesel powered generator and started looking at the stand alones. those kubotas are super sweet but they are air cooled 2 pole units which means 3600 rpms to attain the 60 HZ. (EDIT: I see the lowboy is watercooled, nice machine but at 650 pounds you will need another machine to move it)
Started looking at water cooled, 4 pole units (1800 rpms), but the prices are crazy high for something that may not be used very often. Even considered the Lister types but they are not rated for much over 5kw and are definitely not portable.
The other option I was comparing to was to use a diesel powered tractor that I already owned and didn't really need for anything else and get a PTO genny.
That's the route I went, I got a 10kw pto genny from IMD, which I see now advertises here on TBN. So with the pto gen That solves the diesel power issue, along with portability, water cooled. The only issue I wish was covered is the high engine rpms required to make pto speed, I think mine is ~2600 rpm.
The reason all those smaller portable diesel gen sets are 3600 rpm is that to turn a 4 pole genny you really need at least a 3 cylinder to be able to lug it down that low and still produce the needed torque for the power needed, so you are still using probably the same fuel as when turning 3600 with a smaller engine like Soundguy says, but I still would prefer the slower turning engine.
So for the cost of a good stand alone diesel powered generator you could buy a pretty nice low hour tractor, don't need a loader or 4x4, and get a pto gen. You would find other tasks for the tractor and you wont feel guilty about the generator sitting around doing nothing.
Here's a link to a company that makes the water cooled 4 pole units.
Norpro | A Whole Lot More Than A Diesel Generator Manufacturer
JB.