Simple, choose which song you like better. For me, it's Love the Way You Lie, Part II. The chorus is soooo much more powerful, and I enjoy singing rather than rapping (but Eminem's version is amazing as well).
Here are the links to the songs for people who haven't heard them yet~
Love the Way You Lie (feat. Rihanna)
http://www.youtube.com/watch?v=ZGT6AKt3eYM
Love the Way You Lie, Part II (feat. Eminem)
http://www.youtube.com/watch?v=2B50RUXbs-8