I don't know which Onan you have in mind, but I can tell you always
go with the low speed, heavy duty unit, over the high speed standby unit, if you can afford the cost, volume, and weight (all of which is much greater). The difference is night and day.
I have a Kohler 6.5EOFZ 6.5kW 230 volt generator
on my boat
, and I've been pleased with it. It's very, very quiet and produces very little vibration. If you spend a lot of time on board without shore power
, like I do (my boat
is on a mid-river mooring
with no power), it's makes a huge difference in your daily life whether the daily generator
run shakes the boat or not. My generator is so quiet that it is inaudible from outside the boat (and no splash since I have a water
separator muffler), and hardly disturbs anyone inside.
The difference between low-speed and high speed generators is not just the volume
of sound, but the tone -- the three-cylinder diesels in the low speed ones just lope along pleasantly, with none of the urgency of the high speed two-cylinder ones (I'm not even going to start on the one-lunger Kubotas and Farymanns in some other generators . . . ).
I would also look at Northern Lights
, if I were you. I believe a cut above in terms of quality compared to Kohler, Onan, etc., although as I said, I have no particular complaints about my Kohler, which is a solid, reliable unit.