I have a miller 280 which grew into the 300 a year or so after I bought it. (5 years old now) We have 380 hours on it and the only problem has been the fuel guage, believe the float sunk as it no longer moves. Otherwise vary happy with it and the Kohler. At the time the onan was not a ohv engine and the Kohler also had hydrauic valve lifters and would never need a valve adjustment.
I bought ours online from a dealer out in Maryland and had it shipped to Minnesota. Imported it into Canada. About a month and 100 hours later Millers call me up, says they are having trouble with the armature coming loose on the crankshaft and mine is one of the affected serial #. They wanted it back for updates. I was too busy using it. So they said keep using it and shipped me a new machine. When it came I put mine on the truck and returned to them and kept theirs. I have never heard anything bad about Lincoln, but all my undewear is Miller blue now.
They seem to be a great company to do business withI don't think you can go wrong with Either engine or either manufacturer.
Our first 200 portable ( Canox, built by miller) was new in 86 and the onan on it still runs geat. Other then a muffler and throttle solinoid and one rusty wire connection that overheated and needed cleaned its been great too.
Ken