the types of music are the only differences i have seen. Pandora gives you more artisits very closely related to whichever artist you put in. Slacker will give a broader range of musicians that still pertain to the particular genre
Cool thing about Slacker is you can go to their website, log in, and customize channels by specifying a primary band, then choosing from the other 100 or so it thinks are similar. Go to your bb, update channels, then your custom channel shows and only songs from the bands you picked will play. Only disadvantages I've seen with Slacker vs Pandora is the app size is about 4 times greater and it seems much slower to load and start playing than Pandora.
My vote goes with Pandora. I was using it previously on my PC and now use it on my phone. It uses intelligent software that allows you to rate songs in order to fine tune the selection of music on "channels."
__________________ Red BB Curve 8330m
v.4.5.138 (Platform 220.127.116.11)