different companies go with different brands if you plan on getting a deck that is already satellite capable. Alpine and I believe Sony are XM compatible. I know that Kenwood is Sirius.
But they do have the separate satellite receivers that will work with any deck. I know that XM does, I never really paid much attention to the Sirius. ~Only cause I'm leaning towards an Alpine deck.
They are comparable in price. Down here in Florida, both are pretty popular. Sirius is advertised just as much as XM.
Hope that I helped you out a little cause reading through what I just wrote is confusing me!!