Seinfeld at least went out gracefully. The Simpsons just continues to live on as a shell of what it once was.
At their peaks though, I'd probably give it to the Simpsons.
EDIT : I just remembered how amazing Seinfeld seasons 4-6 were, Seinfeld wins for me.
EDIT AGAIN : Ugh, now I'm thinking about The Simpsons up to season 9. So many fantastic episodes.
I'm so torn.