Sirus has three birds vs. XM's two in different types of orbits. Depending where you live you may get better reception with Sirius. In addition Sirius also has a repeater system in some metro areas.
I have Sirius and I'm very happy with the programming and overall service.
They just added these channels:
FOX News Ch. 131
Become a part of America’s news phenomenon with FOX News, which puts national and international news right at your fingertips. It’s all here: The O’Reilly Factor, On The Record with Greta Van Susteren, Hannity & Colmes and more.
FOX News Talk Ch. 145
From the Fair & Balanced news network comes opinion, news, analysis and listener call-ins, featuring pulse-of-the-nation hosts like Bill O’Reilly, Tony Snow, Alan Colmes, Neil Cavuto, John Gibson and more.
Cosmo Radio Ch. 111
Advice on love, sex, and relationships; tips on beauty, fitness and fashion; the latest celebrity gossip; and the news you need to be the most informed girl at the water cooler. Cosmo Radio takes you behind the velvet ropes with our editors at special events and lets you listen (if you dare!) to guys giving their uncensored opinions on dating and relationships. Plus, you'll hear extra editions of your favorite columns, like Ask Him Anything, Bedside Astrologer, and Guy Spy.
Playboy Radio Ch. 198
Sexy, smart and refined adult programming: Playboy Radio features a full range of original live programming created for SIRIUS, including a new morning show featuring segments with Playboy founder Hugh Hefner, live call-in shows and broadcasts from the Playboy Mansion. Add Playboy Radio at no extra charge.