I tend to use Pandora more. When you enter an artist, I think Pandora does a better job of categorizing music by song.
Slacker only seems to do this based solely on artist. This is bad for artists who crossed over different genres. If I try to build a modern jazz/fusion stationby starting with the band Return to Forever, both services would play music from Miles Davis and Herbie Hancock (two artists who have recorded both traditional jazz and modern jazz songs). Pandora seems to be much more streamlined to playing just playing Davis's and Hancock's modern jazz music. When I start banning those traditional jazz songs on Slacker, Slacker starts to think I want to hear none (or very little) of their music.
Even after a lot of loving and banning I still don't really get the station I want. Of course I am only using the free service; I don't know if some of these problems would go away with the paid service.
For the pre-programmed stations, I prefer Slacker, but when creating my own stations, Pandora has been working out better for me.