I have used both Slacker and Pandora, and I've always leaned toward Pandora based on audio quality, I've always liked Slackers interface alot better, but quality always sucked, just recently there was an update for Slacker, and they fixed a few things with it
-Sound quality is much better! Comperable to Pandora!
-Fixed AVCRP Bluetooth Control, so know you can contol pausing and skipping remotely
-Added caching stations, which is also a nice feature.
Since this update I have gotten rid of Pandora all together and paid for a subscription to Slacker.
I say check it out again if you gave up on it.