FYI, here's the objective differences as I understand them from using them both so far and from the articles linked above and others:
Pandora is computer generated programming based solely upon the music/artists you suggest and thumbs up/thumbs down. There are infrequent audible ads on the PC and none on the phone, just visual ads, which are non-intrusive and easy to ignore. Since I usually listen through my phone, this is nice.
Slacker is both pre-programmed radio and custom - you can select from existing stations (lots of variety and formats, by the way) and modify them as you wish and/or create your own custom station, similar to Pandora. There are audio ads on the phone or PC version. They're short and not terribly frequent, but a bit disruptive nonetheless. One really nice feature the full version of Slacker has, that I don't think Pandora has, is that it will display the lyrics. This is a very cool perk, but available only on the paid version.
Costs for the full versions of both are comparable - both less than $5/month as I recall.
Regarding sound, playback, interface, etc - I'll leave those alone as they are more subjective.
I can honestly see myself using both and may even dump my XM in favor of them. If HTC puts out a car dock that will work with their devices then I'd be much more likely to so I don't have to put it in my windshield mount and then plug in the power cord and the headset line each time. So far, I'm enjoying my Pandora more, since I took the time to "build" it, but I'll continue to give Slacker a shot for a while. They both definitely have their merits.