We rented a Class C with Generac that we used a number of times over the short time we had the RV. It started easily, ran smoothly, and made electricity. We've had two ONAN gensets. The first was a "BFA", ancestor of the Emerald Series. Twin cylinder 1800-RPM MACHINES. Our Microlite 4KY starts, runs, produces too, but I wouldn't call it a "machine." Just something that works. Generac gave me the same impression. A bigger Generac like 5500 might be more like the comparable ONAN Emerald.
Based only on what I've read here, ONAN is the better choice. It may not be all that much better a machine, but offers much wider availability of parts and service. It seems there are more parts on line, more shops willing to work on ONAN but not Generac. Finally with ONAN you can fall back to the professional but expensive Cummins/ONAN truck shops.
Two coaches equal, I'd want the one with ONAN. Better coach with Generac? Probably better off with the better coach, Generac and all.