After Howard Stern & Co defected to Sirius, a takeover of the latter by Viacom seems not unlikely.
The Washington Post runs a comparison of Sirius and XM: http://www.washingtonpost.com/ac2/wp-dyn/A10953-2004Oct6?language=printer

XM Satellite Radio:
launched november 2001, on 2 satellites, in-car deal with GM.
10 $/mo, 2.5m subs as of 04Q2.
content featuring Bob Edwards (formerly NPR) and Opie & Anthony.

Sirius Satellite Radio:
launched 2002, on 3 satellites, in-car deals with Ford, DaimlerChrysler, BMW.
13 $/mo, 0.6m subs as of 04Q2.
content featuring sports (NBA, NHL) and Howard Stren (as of 2006)