I think LeBron has come a long way in defence since his early years where he was known as a below-average defender. But with that said, Paul Pierce is a very good defender himself as we saw this season when he doesn't have to carry all the load, which LeBron still has to. Pierce is also more experienced, so at the moment, definitely Pierce, but over the next few years, providing LeBron get some help, he can be a terrorising force on defence.