Having worked at Sirius and being instrumental in the 2006 2nd generation chip release, here is the full explanation of Sirius/XM.
These two companies were created when a US military EM band was carved up into two bands for commercial use.
*One band was bought by XM.
*One band was bougth by Sirius.
XM decided to go with geosynchronous satellites with the broadcast beam pointed to North America, USA (Primarily).
The problem with this is that the EM is very similar in wavelength to Wifi, so, the signal approaches at a angle (rather than the ideal straight down) and trees and weather can interfear with the signal.
Sirius put three satellites in highly elliptical orbits whereby at there apogee the satellite is directly over the US. While this gives a better signal, it requires 3 rather than XMs two satellites, and also requires a "hand off" as each orbits in turn behind the earth.
Now that the two are a monopoly, I suspect, they want to eventually retire one set of satellites, chances are that eventually will be the Sirius set.
Ok, now with that being said, from what I hear, users are able to receive the Sirius signal. So you should need only to look for a unit that can receive Sirius.
There is a chance with sirius you might notice short dropouts, that should be nothing to worry about.
Note , countries have there own regs on EM bands, so Sirius/XM might not activate when you say you are calling from the DR.