This is my latest comparison after using both for awhile now.
Pandora is better in some ways, especially when it comes to being able to seed a few artist and it sort of picks up what you want from there. You can also select holiday tunes from their normal tunes in station creation. This ability is very much so missing from Slacker. However, with that said once you've used Slacker for a while you get a sense of how it works as well and can pretty much get close to what you can do on Pandora, though for calling itself Slacker I have to say Pandora allows me to be a much bigger Slacker. Also like I stated above the ability to listen to the quick mix is great. An example of this is I can mix Christmas Country, Christmas New Age, Christmas Christian, Christmas Traditional into a Christmas Shuffle Station and it is GREAT!
What ultimately sucked me into Slacker was the portable unit. Even though I do have Pandora on my iPod Touch, which I listen to almost every night. Since Slacker portable doesn't give me any means of doing a sleep mode, I have just come to the conclusion that my best bet is to use my ipod Touch with the Pandora Application. This works great! I also still like the Pandora New Age and Meditation Channels I created better than their Slacker versions, but mainly because absent from Slacker are many of the more deep new age and meditation artist. Also Pandora uses a lot of various artist albums to draw from and Slacker does not.
So there you have it, they both have their strenths but ultimately I use them in differently. I do find that Pandora repeats a lot more than Slacker and If Pandora had a device that allowed me to download their stations to talk with me on the go that would be great. But they seem to want to be a streaming service, where Slacker is about downloading content to the device and having it readily available on a device for playback. Fact is I enjoy them both and have a hard time just saying one is better than the other. But I give Slacker the edge due to the portable device and the fact that I don't need an always on live connection to the internet to listen.