Having owned both, and played both of them a TON... I'd go with Rush the Rock, just for the depth of the game. More tracks, lots of secrets, shortcuts, etc... I think you'll get more long-term enjoyment out of them, and if you have friends over, that's probably an easier one for anyone to jump into.
Super GT is an amazing game as well (one of my favorites), but it's a more "serious" arcade racer. It's really all about technique to master the game. The reason I finally sold mine was that I was able to consistently get first place on all the courses with the F1, so every race was really just about making the perfect run without screwing up. I got a LOT of enjoyment out of it though... I had it while I was going to college, so every night that I'd be up late studying, I'd take a break and play Super GT for a couple hours.
The only reason I sold my RTRs was that I picked up a pair of Rush 2049 SEs. I really hadn't mastered the game, though I played a ton of it. And friends definitely enjoyed RTR more.
DogP