A couple of other differences, then, lolol:

Sirius has a lifetime subscription for $500, XM's highest level is a 5 year term for the same price. With Sirius, you get free satellite radio on your PC, XM has it for $4/month.

Channels:
Sirius = Howard Stern
XM = Extreme XM

Sirius = Maxim
XM = Playboy

Results comparison review:
Sports = Sirius - based on wide range of channels
News = XM - has top brand name newscasters
Talk radio & Entertainment = Sirius
Hip Hop/R&B = Both
Pop = XM - very good mix
Rock = Sirius

Later, QD.