BRP.
It seems players get more attached to their d20 characters, and actually expect to... LEVEL UP with them. Can't run a 3-year campaign with THOSE expectations. My 3-year BRP game had an average of 25% casualties per module. On the other hand, it was VERY rewarding to be able to reach into the d20 book and pull out stats for a pair of Shoggoth.
Nyarlathotep vs GnarlyHotep the Surfer with a Thousand Toes and the Fun Guys from Yuggoth.