I played the demos for both (Blur multiplayer demo is available now). I wasnt thrilled with S/S, but I loved Blur enough to buy it. I've been playing it since.
Blur wasn't meant to be a sim racing game, its arcade for a reason. Its the kind of racing game that once you are out in 1st place, its not a sure-win. Your work is only just beginning once you make it in the front of the pack.
The post saying "NFS meets Mario Kart" couldn't be more accurate, lol. I only wish there were more car customization, but there are a TON of cars in this game.