so... basically all programming is available on both... but you have to pay.
For instance, Stern is free to Sirius subscribers, but cost $2 month for "Best of Sirius" package on XM.
If you have Sirius, you can get "Best of XM" which gets you Oprah or some other useless crap.
The ONLY exception I know of is that the MLB baseball package (where they air all the games) is ONLY available to XM subscribers.
Also, if you have an OEM Sirius unit, you cannot receive the XM channels as the BMW module can't decode it (hence the new module coming out soon).