sirius hands down. had XM before, programming sucked.
sirius = NFL + NBA, awesome rock stations, awesome rap stations, awesome country.
xm = mlb, new contract w/ the NHL, and a good chunk of the college games. i believe they have better coverage as far as reception goes. (from what i notice driving up and down 35 and in denton) i could just never find anything i really liked on XM i was constantly having to switch the station.
i will vouch their comedy channel 151 is pretty good, i havent listend to channel 104 (sirius' comedy station) yet to really compare.