South Park reinvents itself every year, and the Simpsons are still on the "Ssssssure it is" joke. I would watch the Simpsons over South Park anyday, but at the moment South Park is the fresher show. Getting rid of Kenny was a smart move, as smart as changing the Simpson focus from Bart to Homer.
As far as classics, the Simpsons will be the next Brady Bunch: always on the air. I highly doubt that South Park would have reruns after it's over. Although newer episodes are very stale, The Simpsons is clearly the winner.