Disclaimer: I've not driven the latest version of either Sim, my observations are from a couple of weeks back.
For me iRacing AI in close proximity is better than ACC, they fight better, chase you down better and generally keep their line better. ACC AI are somewhat timid, too afraid it seems in certain situations and also too train like and only overtake if you make a mistake.In iRacing they often make more human like errors/mistakes which gives it another level of realism, ACC on the other hand i rarely see them off track.
I'm eager to try both latest versions, especially the oval AI in iRacing but that's another story!