Yeah, Sirius teamed up with Audiovox (ha ha) and Kenwood to challenge the Skifi by Delphi. So, if you wish to add a satelite radio system to an existing oem radio (although i really recomend replacing your head unit) you can do so with either satelite provider.
Which one is better, that 's up to you. The content each provider offers differs slightly requiring you to go to each website, compare the channel line up. Sirius is $13/month (but are currently offering 3 months free with a one year agreement) and XM runs about $10/month. I have Sirius and am very happy with it, here is my outboard Clarion Sirius Receiver with my Nakamichi head unit.