Its not even a contest. Slash is one of the only guitarists from the last 20 years to remain relevant and become a recognizable guitar hero. That didn't happen by accident. Every solo he plays like its his last. You can feel the emotion there.
While I like Morello and his playing, its fairly one note and his big "move" is the scratching on his guitar, which I'd like to say isn't a gimmick, but it tends to show up in everything I hear from him, mainly in his guest appearances. Solely because its his "thing" and it impresses people during guest appearances cause they've likely never seen it. But it gets old fairly quickly. He is a good guitarist though.
As a huge fan of Audioslave (never listened much to Rage Against The Machine), I LOVE the Morello solos. I know many hate it, but there is something mesmerizing about them. But technically, I'd have to go w/ Slash, but I just like Audioslave songs more. Probably a lot also has to do w/ Cornell on vocals -- amazing!!