The Generac generator is belt driven from the gasoline engine. Generac 5500 engine rpm 2571, generator rotor rpm 3600. With a belt driven system the potential for belt slippage under load is present, along with the need for belt replacement over time.
The Onan 5500 is a direct drive with engine and rotor rpm 2400. My personal preference would be either an Onan or Kohler over the Generac unit.
Both Onan and Kohler have been in business for many years and have a relatively good track record.
Good luck with whichever unit you choose..