The USA SPEC kit and DICE have the same core functions (steering wheel/radio control), iPod charging and direct iPod connection), the DICE goes a bit further in features:
- Text display on your radio or navigation display
- unlimited Playlists (BMW kit is limited to only 5)
- ability to browse by song, album or Playlist
- Full control of iPod's Clickwheel (BMW kit locks out the wheel)
- Retains display on iPod (BMW kit places a logo in place)
- Integrated AUX input
- All iPod features are still functional (highest rated songs, play count, on-the-go playlists, etc)
As you can see, the DICE kit is the better choice.