I can't really comment on the RS5. I know the PS4S is the tyre of choice. Which I have 4 of them on now. Previously I've had 3 sets of Rainsport 3s. TBH, I can't really tell any difference grip wise, straight line or cornering between the two. I think next time I might give the RS5s a try.
The only thing I can say that is worth noting (again, not sure on the RS5). But the PS4S stick out past the rim (RS3 don't). I guess this is kind of a bit of kerb protection, although I wouldn't like to test it. The only downer I've found to this is because I have coilovers, the PS4S rubbed and I had to raise the suspension up 10mm and it probably needs to go another 5-10mm. Whereas with the Rainsports not sticking out as far, they always missed the arch.
Dunno if any of that helps. LOL.