Sirius' sats use a different orbit which place them higher in the
sky, allowing them to get overtop of buildings better than XM, so you'd have
slightly better luck with Sirius. However if you're planning for mainly in
home use, XM is a bit easier to receive at the house, due to the signal
coming in from lower on the horizon, through the windows rather than from
straight up.

Their satellites are in a 24 hour orbit, but not in the usual
geostationary orbit. They have 5 or so satellites tracing what
looks like a figure 8 path in the sky, 2 or so are pretty much
overhead the USA at any one time. Elsewhere in their orbits
they go silent and just recharge their batteries. This means
that there is less tall building shadowing, and they didn't have
to fight for space on the usual geostationary orbit.