Seagypsy, in some ways your choice is a bit like choosing between a Commodore & a Falcon. They both do the same job just in very slightly different ways. I went thru this decision loop a while back
First suggestion is get out your tape measure and see if they both fit. The Nanni is a 4 cyl whilst the Yanmar is 3 cyl and the Yanmar´s gearboxes are very compact, so the Nanni typically requires a little more length.
The Kubota block which the Nanni is built on is bullet proof (look under the hatch
of any bobcat) and with its indirect injection will be a little quieter than Yanmar. However I preferred Yanmar for it direct injection because if your battery
is a but iffy you have a better chance of getting it away.
Both use aluminium bodied Heat exchanger/exhaust manifolds and this is probably the major maintenance
issue. A problem I have been advised with the Nanni´s (but not verified) is Nanni tends to change it suppliers frequently so the apparently same model may have different components from year to year.
The Nanni has a greater PTO allowance if you want to run a big alternator
. Also check exhaust
sizes, my 4JH4 required a 75mm exhaust
which is just ridiculous for a 2.2l motor.
There have been problems with the latest 4JH4 Yanmars which have an electric fuel
transfer pump, not sure if this applies to the 3JH4(search this site for discussion).
Finally if you decide on the Yanmar, forget about the standard isolation mounts - suggest install the aftermarket polyurethane
mounts from Polyflex in Brisbane
At the end of the day both are good motors, it will just be the little items that swing your decision