I currently have a Sirius Sportster 3 and was considering buying a Sportster 4. Is there any difference between the two units other than the case?

On a related note, which Sirius radio has the best reception in a car? My Sportster 3 has pretty spotty reception.

The only difference between the Sportster 3 and Sportster 4, is that the Sportster 3 does not have the 44 minute buffer and the 4 does. Receivers do not have an impact on reception, antenna placement and geographical location do.