If you are into Howard, Bubba the Love Sponge, and Maxim. I guess Sirius is the way to go.
If you are more news oriented XM has a slightly better offering.
The music is about the same.
Both companies are competing heavily and changing programming choices. Sirius just got NASCAR, XM has baseball. Sirius has Martha, XM has Oprah. If you chase the programming, in six months the other service will offer something that you wished you had gone with them.
Both companies need to make some profit or there will be a lot of angry customers holding the "bag" (receiver).
Me, XM. You, it's your preference. Get the service that suits your needs and don't question your decision.