Having had to make exactly this decision recently, here's what I did..
Budget left me the choice of Quest vs Rift S. My main driver was also sim racing games, which would have meant Rift S, however I did want to be able to bring it downstairs so the family could have a go, and the Link/Virtual Desktop feature meant I could play rift games as well. So I went for the Quest, and haven't regretted it.
Having not seen the Rift S display it's hard to be categoric, but the display is 'OK' on the Quest when playing Assetto Corsa and Project Cars 2. I don't expect it's that much better on the Rift S. Resolution is definitely the one area that's lacking, but I think this is a general VR problem at the minute, and I don't think paying more than double for an Index would give improvement enough to justify the outlay. What's lost in resolution is more than made up with in immersion and depth perception, it's a night and day comparison with racing on monitors for me. Lap times have also drastically improved, but that might just be because I'm playing more
So it works for me, and at 72Hz it's fine for Sim racing. Definitely could be better, but it works. We also have a good laugh playing Beat Saber etc and it's great that it's not fixed to my PC, so there's that as well.
For me this was a 'test the water' in VR. I'm most definitely hooked and am now eagerly awaiting the next offering, this new HP/Valve HMD looks like it might fit the bill...