iRacing is awesome. Race2Play is another one to take a look at since they have over 250 cars and more tracks.
I have some experience with iRacing (not my rig) and the physics model is impressive. The graphics are very smooth and car models 100% accurate. The reason they do not have as many cars is because they aim to model them accurately with manufacturers CAD drawings and months of R&D. They even have live NASA marshals watching the racing and dishing out black flags if you are acting fool!
I can't beleive they have Mosport! Now I want to get iRacing.