Inverter generators are more efficient, quieter and should all provide clean power, as in a constant 120v as 60hz. Non-inverter generators are louder, less efficient and often do not provide clean power, especially as fuel is running low. (Not really an issue for natural gas)
I do think it is important to look for a generator that provides clean power.
The fuel efficiency issue is more a matter of pay now or pay later decision.
The low noise is a really nice feature.