It depends on the current system's in your boat and car/truck. Most audio companys have partnered with primarily XM in the past couple of years, and have many satellite ready head units.
Most of these have add on receivers that plug into the head unit through the auxillary bus port or through an RCA input eliminating the need for the RF adapter and display the information on the HU's display panel.
The RF adapters are there if your unit is not satellite ready and you dont feel like investing in an upgrade.
I believe Pioneer or Alpine (I'm not really sure) has a transferrable XM unit that would allow you to do what you want, while maintaining the signal quality. (But I suppose you would need a docking port in all the places you plan on using the receiver, this may or may not be cost effective)
As far as XM versus Sirius, I have neither, so my opinion here is as worthless as any internet wire jockey. :smile: I have seen the playlists for both companies and they seem very similar, as do their pricing. furthering with my opinion, I would have to jump on the XM bandwagon as they seem to be the one making it big....most auto manufacturers as well as aftermarket audio have gone that route and thats gotta put the squeeze on Sirius.
I do have a Kenwood HU, Kenwood opted to partner with Sirius, so thats my option....
Dont take any of the preceding post Siriusly though, as I have no clue what the hell I'm talking about.... :idea: