iRacing is by far the most advanced racing software and the same one used by most professional racers and has the best racing dynamics which is why we use this as our main program at Race to Reality. We have the commercial version so we have every car and track on their database. (Our Full Car Simulator currently only runs on iRacing)
In short iRacing is our first choice but we can put you on Assetto for the following reasons:
Non nationall track like Thunderhill, Pikes Peak, Spring Mountain
You want to drive either a street car or a car with a H-Pattern, (iRacing does have a few H Pattern cars such as NASCAR Xfinity, older Miata, 90GTO Audi, NASCAR TRUCK, LATE MODELS)
Drifting is also on option