Your slacker subscription comes free with your tesla, you shouldn't need to cancel it. Spotify in the car on the other hand requires a premium (paid; not ad-supported) account. I think this is how they're getting around the contractual obligations to slacker, with a bring-your-own-account model. Works out great for us who already have a Spotify account and use it on all of our other devices.
As said before though, if having "radio stations" based on artists/genres is all you want, Slacker will be fine for you. If you want the ability to play full albums, listen to the same track 6 times in a row on repeat (hey, don't judge...), or have playlists autogenerated based on your listening trends (as well as ones you create yourself, or ones curated by other users), with a larger library, then Spotify is worth the extra couple bucks a month.
