I prefer Pandora by a longshot, but I have pretty underground music tastes. Pandora actually notices that most music you like on a particular station is in a major or minor key, at a certain tempo or having a certain texture. Slacker just plays music for you and takes note when you like or don't like a song or artist. Slacker doesn't know the difference between techno and electronica. I have a minimal techno station on Pandora. The closest thing to it on Slacker plays crap like Daft Punk and the Chemical brothers. I have an underground hip-hop station on Pandora that plays artists I've never heard before. I didn't hear a single track I'd never heard before when listening to hip-hop on Slacker. Slacker's also a HUGE space and resource hog compared to Pandora. But on the flipside, Pandora can be repetitive if you don't take the time to really build up your stations using the full flash site from time to time.
There are TONS of threads on this topic though, and you'll see that people seem to be pretty split as far as who prefers which app.
