Some content like Stern and NFL are Sirius exclusive and if you have XM you have to pay extra for what they call the Best of Sirius package to get it on an XM tuner...the same goes with XM content like Opie & Anthony and MLB you have to pay for the best of XM package on a Sirius receiver.
the music content/channels are the same you can see their pricing and packages here:
http://www.siriusxm.com/ourmostpopularpackages-sirius