Basically if you want to drive GT3 cars, you should only drive in ACC and only on its tracks, and no point doing that anywhere else. If you want to drive in places they don't have, too bad take a hike.
I used to barely touch ACC over the last months and gave it a try this morning. As a result, I uninstalled it!
Like everyone here basically says, it's just personal preference. And I drive/race almost 100% with GT3 cars. Yet I enjoy AC way more than I ever enjoyed ACC.
ACC still has outdated circuits like Zandvoort or Spa. And every time a new DLC or huge update comes out I get disappointed. At least with the US DLC we got Watkins Glen which I like, but still, no Road America for example. And now from all the circuits out there we got Valencia...!
Also, for me the physics in ACC feel like ****. Maybe it is because I'm used to the cars I use in AC, I don't know. But they feel very strange. Even gave it a couple of tries after the last update. But if anything, it felt even worse to me!
That's why I've barely touched ACC the last half year and after uninstalling it today, I'll probably won't ever touch it again.
On the other hand, in AC you can have more or less anything. Many circuits look like ACCs in quality nowadays and the cars feel really good too. And there is this amazing community behind it, like this forum. Every time a have a question about anything I'll get a solution within minutes or hours.
So before you send us all hiking again, ask yourself why racing with GT3 cars around Road America, Sonoma, Long Beach, Nordschleife, Red Bull Ring, Motegi, Chang and so many other great circuits is not possible on ACC. And it is with AC. And it's actually lots of fun!