lol? Wow, nobody answered this? That's rather shocking. I play both. I have about a year in ACC, and a bit over 2 months in iRacing. That said, I can tell you that there are only two reasons I still play iRacing:
#1 I've spent over $100 on the content I currently use, and don't want it to have been a waste.
#2 iRacing does multiplayer far better than ACC, and I'll expand on that statement: ACC has a very limited and, frankly, vague multiplayer functionality when you're starting out. It took me forever to figure out why I could only ever find servers for Monza. Originally, I thought nobody ever raced any other tracks except the pros, and then I realized you don't even see the other servers until your ratings are high enough, which I think is just complete nonsense. They should at LEAST give a detailed explanation for how the server systems work instead of leaving us to guess and scour Google for answers. iRacing's multiplayer functionality is very straight forward and self explanatory. It does have a sporting code you need to take time to read(unless you wanna get banned because you didn't know about the rules), and the rules are very strict, but the strictness of them makes for a very competitive and clean racing environment, so it's very, very worth it. Aside from that, you can almost always find a good, competitive race in iRacing because of how it all works. You choose a series in which people are currently racing, and register for a race. The system then takes your iRating into account, and matches you up with similarly skilled racers by putting everyone on different servers called "splits." Races in normal series happen once every hour, 24 hours per day, 7 days per week(unless there's down time), so you can always find a race, no matter where you are, or what your schedule is. Every series is on a season schedule. For example, right now, iRacing is in Season 4. Every week on Monday night(CST American), the track changes, so you get a new track for that series every week. Right now, the Ferrari GT3 Challenge is racing on Spa. Tomorrow night, it'll change to Road Atlanta. In short: iRacing's MP system simply blows that of ACC out of the water, period.
All that said, here is why ACC is a better game: The physics, graphics, sounds, and track accuracy, are all better. The cars in iRacing(especially the GT series) are very agile, and not nearly as dependent on things like setup, tyre temp, etc. as they should be. The sounds in iRacing are better than any other sim except ACC, but ACC far out-does iRacing in this area. The graphics... lol... comparing ACC's graphics to that of iRacing is like comparing Cyberpunk to Skyrim. There's just no competition. I will say this though: ACC suffers from blur, and iRacing is a lot graphically sharper than ACC, and this helps a lot in VR(Virtual Reality, I race exclusively in VR), which means the tarmac honestly looks a lot better in iRacing than it does in ACC, but when it comes to literally everything else, there is simply no contest. ACC just obliterates iRacing graphically. iRacing isn't even compatible with supersampling, which is just crazy for a game that supports VR(SS is ESSENTIAL to VR). You have to go into the game's files and manually turn the pixel-per-eye WAY up just to make the graphics tolerable in iRacing VR, which severely hurts performance(I get similar FPS in the two games because of that, which says a lot considering how much more intensive ACC's graphics are). But, after you do that, iRacing doesn't look bad, even in VR. I've seen a lot worse(like rFactor 2). When it comes to tracks, ACC and iRacings are very similar in terms of accuracy. The layout of them is exactly the same, except for topography. iRacing is off in that area often, and ACC is, for lack of a better word, just perfect.
All of this is, of course, ignoring the fact that even if you ignore the $13 per month you have to pay just to play iRacing, it still costs 6 or 7 times what ACC costs, and ACC is simply a better game in the GT world. However, I'd say both are worth playing, if you're willing to spend the money, especially if you want more than just GT racing(iRacing does NASCAR, Formula 1/2/3/Vee, Oval racing, Road racing, Dirt Oval, Dirt Road, GT1/2, GTE(Endurance), GT3, GT4, Mazda MX-5 Cup, Trucks, V8 Supercars, you name it). But if you're just looking for the most realistic sim you can get your hands on, regardless of the driving discipline it features, then ACC is the absolute best, hands down.
Edit: I forgot to mention that ACC's AI racers are far superior to that of iRacing.
Edit #2: I completely forgot 2 important mentions: #1 The Force Feedback in iRacing is a lot stronger, but perhaps a bit less detailed, than in ACC(I honestly like it stronger). #2 There's a 3rd reason I still play iRacing: It's because there's some kind of stupid bug in ACC that I seem to be the only one who has that prevents me from choosing which tyre set I want to use in the setup menu. It automatically changes to the next tyre set in line each time I start a new session or go to the garage, but I can't tell it to use a different one. When I do, it just reverts back to the original one when I exit setup. That is pretty much game breaking for any race that requires more than one session, or requires me to tweak my setup by going back and forth to the garage, unless it has unlimited tyre sets. ACC's technical support is ignoring me, and their forums are a joke, so I haven't played ACC in a few months beyond some random practices for like 15-20 mins from a random urge I may get to play it.
