I've bounced between both, Sports Mode seduced me back to mainly GT7 the past few weeks, although since penalty system seems to have pretty much disappeared, I haven;t played Sports mode for a few days now, but I'm enjoying usign various Praiano tunes for all sorts of cars and races.
As has been mentioned ACC is 'free' now with playstation + premium tier, although assume may have to pay extra for the 3 or 4 dlc packs/? Still, I only paid around £12 in Ps store sale the other month, then within hours bought the packs for around £40ish total...
ACC 'feel' is incredible, ffb is another dimension. 1st 30 mins or so took some adapting, but once it clicked I recall thinking 'wow, this is a phenomenal experience'. Menu's are a bit more clunky, lacks a bit of polish compared to gt7 sheen, but who cares when the driving is good. I ended up doing loads of laps over and over at Brands Hatch (you kind of have to do a mini intro, passing tests like doing clean laps, push the car more aggressively etc before it opens up more of the game. All the sounds, car bouncing around and even the team mate in the pit giving feedback over the radio at end of each lap adds to the occasion.
You can toggle AI cars performance/difficulty from 1 to 100, also 'aggressiveness' scale too... so its really granular, you can incrementally change the setting as you get faster, or reduce it if trying new car or track. I had perception it would be difficult to get anywhere from stuff I was hearing, but with the safe tune,
Also a massive + for ACC is cockpit view (with or without a wheel visible), where you can change FOV and a load of other settings to get it spot on for exactly how you want it. Or you can use a FOV calc online which advises optimimum settings, depending on distance you sit from screen, size of screen etc.
I can understand why some think/say its complicated, but from googling around, you don;t need to worry about much apart from getting view setup, wheel ffb and the odd straight forward option. The cars come with a safe and a aggressive tune, both of which are absolutely fine, the aggressive tune can get you strong times. There are websites which advise optimum tyre pressures/temps for each car/circuit and full tunes for each circuit. TCS ABS options/parameters vary from car to car and are based on real life setup. Some cars have 2 tcs settings. one is for how quickly tcs kicks in and the other for how aggressive it is once it does... again, apparently based on real life.
Gist I got was that the default tunes are absolutely fine for most, and its only when trying to get final few tenths out of lap times, tuning becomes more important. Much more to be gained by spending time with the car on the track.
Cars feel more closer to 'real' behaviour in my opinion. wHen you spin or make mistake you kind of know why, whereas in gt some cars behaviour just seems unpredictable/out of the blue.
I haven;t tried career mode, sounds kind of interesting/weird, with random races, challenges... not sure if they change each career you start. Need to look a bit closer. I usually prefer racing in games, but ACC is really engrossing, rewarding, just lapping over and over by yourself. Feel really immersed in it, get into a rythym/state of flow and just driving alone is a good experience tbh. I've pretty much only used Brands Hatch on the game and BMW Gt3 and 4 cars and have enjoyed just lapping race with them on the one track, which prior to ACC, is not how I've ever played racing/car games, preferring to try loads of cars and races ASAP.