I played Driveclub today (three hours ago).
That thing is aliasing city.
It seemed to have more stuff going on in the background with the track they had (such as trees and bushes), but even the car's dashboard was aliased to hell, and the painted lane lines on the road were shimmering as they were coming up on you. The powerlines that went around the track weren't even visible because of the heavily aliasing in the distance, whereas that was way lessened in Forza. In my opinion, Forza had way, way better image quality. Car models looked way more realistic, aliasing was nowhere near as noticeable (didn't ask if they used AA or not though), driver hands looked incredible, etc. I've also seen some videos of Forza going through foresty areas, but I didn't get to see that in person, so maybe Forza will be able to match Driveclub in the background effects will still having good IQ.
Regardless, I'll still try out Driveclub PSN+ when I get my PS4 because of course they'll be improving the game up to launch, but with the 5 minutes of hands on time I've had with the games, I've decided Forza is getting my money this November.