XM/Sirius also uses terrestrial repeaters, so you don't necessarily need line of sight to the satellites. I have an XM radio in my office at work, which is an interior office under a staircase, opening onto a glass-covered interior atrium. Although it's not really strong, I rarely lose signal.
As an aside, despite the fact that XM and Sirius are now the same company and share most of the same programming, the respective services use different sets of satellites and repeaters. I recommend getting in via the XM part. The two XM birds are fixed in the sky relative to Earth (geostationary), whereas the three Sirius birds move up and down in the sky to below the equator/horizon over a same general elliptical area (geosynchronous), meaning that their angle to the listener changes throughout the day. In theory, one or more of the Sirius birds are visible at any time, however when I had a Sirius radio in my car, I found that later in the day, I seemed to often lose signal, something I didn't/don't experience with XM. The nearest mountains are 150 miles away, so that wasn't a factor. Also, XM has or had more repeaters on the ground than Sirius.