Onan used to make the best generators....cast iron block, 1800 rpm units. They sold that unit to cummins......not so good any more. I believe onan kept the RV line, but these may just be rebranded cummins...i really dont know as i do not service rv units. Generac has something like 90% of the residential market. Alot of the people bitching about generac units use an air cooled unit where they should be using a liquid cooled. They take a unit designed for emergency ..... occasional.... use and try to use it for primary off grid use. Doesnt end well. I used to work on cummins, briggs, generac and even some Catipiller units. I moved out of all the others because i was getting better access to tech support and parts with generac. I service 100’s of generators every year, and i really have very few issues with the generac units. When they do fail the parts are usually easy to come by and relatively inexpensive. Until you get to the rotor and stator. Personally i have never seen a failed one. But if the go....almost cheaper to replace unit.
I love the diesel generators, but i can only count on 1 hand customers willing to pony out the $$$ necessary to buy one. Here power outages happen, but are too rare to have someone spend up to 4 times the cost of an air cooled unit for a large diesel and fuel tank that meets epa storage requirements for use over the aquifer where i live. The last commercial diesel i was trying to put in had a requirement for diesel containment that would have outpriced the generator had i gone thru with the install.
The only think i like about the Kohlers over the generac were the hydraulic valve lifters. I HATE having to adjust valve lash.
But now generac 13 & 16 KW units (soon to be 14 and 18 kw as there increasing alternator sizes on that line) have hydraulic lifters. I personally do not find any more reliability in the actual generator between the lines. A 3600rpm screamer is only ment to be a backup power source. They are all about equil in reliability in that respect. There really is t much different in the mechanics between the lines.
Generac puts the system brains in the generator and the transfer switch is dumb. The rest seem to have smart transfer switches and dumb generators. Both work just as well as far as im concerned. During the last power outage here i received calls on both kohler and cummins units not working, along with 2 calls on generac units. I was able to resolve the 2 generac unit issues over the phone, but i told the other people that i no longer work on those brands. I dont have parts for them and have not kept up with their service manuals. So any brand can have issues.
My personal recommendation is to buy a brand that has service reps available in your area....and i mean a few different companies not just 1 guy that might be retiring (by the way im retiring) . I get calls about Generac units needing basic service that are 4 hours away from me. They say there are no local reps......not a good thing. And im not driving an 8 hour trip to service one. If you have no reps around, id say skip the whole house and go portable.