Both of these games bring a refreshing change of pace to the racing genre, but I don't know which one deserves my money. I played the Split/Second demo, and it was enjoyable, but seemed far too easy. Blur looks like its multiplayer would dominate, but I like the story behind Split/Second better (although Blur's reminds me somewhat of NFS Most Wanted, my favorite racing game of all time). If you have played both, please contribute to the thread.
Blur is amazing. Single Player is alright, but the multiplayer is the delicious cake/pie. Blur for replay Split/Second if you love disney.
Personally, I'd choose Blur since I know it is great, but Split/Second looks good as well. The main complaint about S/S I've heard is that once you've played tracks a couple times, you know them so well that you dodge everything. Blur has random power ups and always keeps you guessing, and is truly the Modern Warfare of online racing with all the challenges, cars, and unlockables.
I hate to join amongst the sheep, but Blur. That is almost exactly what I was going to say. The developers even cited Modern Warfare as one of their inspirations for the multiplayer, and it shows in Blur's quality. Realism meets Mario Kart... e.c.t, e.c.t.