If you want to do properly organized multiplayer races, Iracing hands down.
Saying Iracing physics are not as good as RF2's is not accurate, they both have certain aspects which they do better than the other, but overall both are very good, and its minimal differences, for example the drag generated when bottoming out and the car's reactive behaviour to it based on the impact can differ quite a bit between RF2 and Iracing, will it break the experience or make one worse than the other? absolutely not.
I made my own car for RF2, so I have a bit of experience
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