I downloaded the two to my cell phone, and here's my thoughts:
Pandora's not bad, especially if you have a favorite band. I typed in "Nine Inch Nails" and every other song's from Trent, but I also heard other bands somewhat tied in to NIN like Marilyn Manson and Johnny Cash. However, I also heard John Mellencamp's "Jack and Diane" on my station. Huh?!?! Plus, what's with the obscure ambient crap like Zero 7 mixed in?
However, I like Slacker much, much better. It's more like Sirius since there's a variety of channels and artists, and their 80's alternative station's quite awesome. I've given the 80's and 90's alternative and the hard rock channels good listens and even though I got rid of a few artists from these stations, they're all good, IMO. I even like the fact that the hard rock channel does a great job with new music. They might be better than Octane, which has gotten stale with lame-o chick rock and obscure junk. However, hearing PSAs every three songs and watching an Olay ad because I changed the station can get old.