XM
XM Radio comes standard in most new-car models from Acura, Isuzu, Buick, Porsche, Saab, and Volkswagen, and from GM brands Cadillac, Chevrolet, GMC, Oldsmobile, Pontiac, and Saturn. It's available as an option with models from Audi, Honda, Hummer, Infiniti, Lexus, Nissan, Scion, and Toyota. While the cost of the options package varies from company to company, the average price for adding XM is $325.

Sirius
Sirius is available in Ford's Jaguar, Lincoln, Mazda, Mercury, and Volvo brands; Daimler-Chrysler models, including Chrysler, Dodge, and Jeep vehicles; and BMW, Land Rover, Mercedes-Benz, Mini, Audi, Infiniti, Nissan, Porsche, and Volkswagen models. Check with your local car dealers to find out which models have Sirius standard or as an option, and about installation pricing.