I clicked ACC, but it's close, both have pros and cons. Looking from a purely offline perspective in what follows here.
ACC looks better, but being a VR player, performance is much worse. I think rF2 looks and runs better in VR, given the amount of settings that need turning down in ACC. Some people don't like the way rF2 looks, but I think it has quite a natural feel to it. Perhaps not as detailed as ACC, but it's capable of looking very nice indeed in the right lighting conditions.
Driving similar cars in each, I wouldn't say one is better than the other. Other's may get hung up on physics and FFB, but they both work well enough for me. Driving all the major sims, I doubt I could tell which physics and FFB belonged to which game in a 'blind taste test'.
AI is where rF2 prides itself, but so much is missing or flawed as mentioned above, I don't run weather, or pitstops. It's very good on short races though. Mod tracks can let it down at times, with poorly tuned AI spinning or taking weird lines. You've got to be careful about which car and track combos you choose, but when it shines, it really shines.
The ACC AI is good, but nothing fancy, they don't seem to fight very hard to save a position, in fact they almost wave you through sometimes.
The new UI in rF2 is horrible, I hate it with a passion. It flickers, stutters, lags and just doesn't look good to me. Sometimes it crashes and puts up an error message behind the game window, so sometimes I'm sat waiting staring at a loading screen, not realising it's had an error. I wish they weren't retiring the old, less flashy UI, for me it's much more functional although it does have it's own problems. Also the more tracks and cars you have installed the longer it takes to load up as well. I keep my cars and tracks in a different folder and transfer only the ones I want to drive into the game folder.
The ACC UI works well, isn't anything fancy, but it's all nicely presented and has lots of info.
I play most of the major sims, but I wish rF2 fully lived up to its early promise. If they sorted out the AI bugs/flaws, the horrible UI and had an offline championship mode (in game) I wouldn't need anything else. Sadly, if frustrates me so much at times, I go weeks without playing it.
The prices of the DLC present better value in ACC to, but it's a shame we can't run a custom championship with it. RF2, thanks to the excellent rFactor2 log analyzer makes custom championship easy, but really after all this time, we should be able to do it in game.
ACC, whilst perhaps not quite as good a racing experience as rF2 (if the conditions are right), I think it's a more professional package when looking at the whole product.
