Sirius has several DJs from KNAC that I listened to when that station was on the air.
Otherwise, their programming is about the same and it helps that Sirius's Hard Attack has shows like Ian Christie's Bloody Roots, Into the Pit, etc. They also seem to have no qualms about playing any bootleg recordings (i.e. Rob Halford singing Black Sabbath with Black Sabbath when Ronnie James Dio refused to perform).
Sirius also has Howard which is the reason that pushed me in their direction.
The show titled The Rock of Jericho on XM Radio on The Boneyard, its on sunday nights or something.. but his show is fucking awesome, the guy really knows metal.
SIRIUS is by far better for metal. Hard Attack is great. They definitley play everythinig, and they play new stuff very early long before its released or even leaked. They were the first place that played new Opeth before anything leaked.
Plus SIRIUS has Howard Stern, why would you even bother with XM?