XM has two satellites in geostationary orbit - this means they're above the equator. Since we're in north america, the satellites are always south of us. Any time you are on the north side of a tall obstacle (large building, an east-west valley, whatever) there's a good chance you won't be able to see the satellites. XM makes up for this by having about 1,100 ground based repeaters.
Sirius on the other hand has 3 satellites in a geosynchronous orbit. This is a hard orbit to explain without a picture, but all 3 satellites are on the move (not stationary) in a figure 8 shaped track that loops over the US, crosses over itself down over central america or so, then loops again south of that. Only 2 satellites are over the US at a given time, the 3rd is in the lower part of the loop.
The net result is that the Sirius satellites are generally more directly above you at any given time. The reciever also has two to listen to, and those two are in different directions. Obstacles are less of a problem. Sirius only has about 95 or so ground based repeaters, because that's all they need.
What's this mean to the average person? Probably not much if you're in an urban area in the US. The only significant difference is that the ground based repeater network costs a decent amount of money to run so it's a drain on XM's resources (but they have a lot of money). Sirius has a bigger footprint as well, but unless you live way up in Canada or in Central/South America, that doesn't matter.