I know more about satellite radio than I ever thought I could because I deal with it all day every day for work. No, I don't work for either of these companies.
Comparison:
They basically offer the same genres of music. Generally the same numbers of channels. They differ on some of the talk shows and on exclusive sports (XM has World Cup, MLB and NHL; Sirius has NFL and NBA). Both offer their service in Canada. Howard Stern is obviously Sirius only. I hear Opie and Anthony are ok, but I don't listen to either.
XM's playlist for the music channels is generally considered deeper. That is, they play less well known songs and a greater variety (some people really like this because it develops a greater knowledge of the genre by exposing you to music you wouldn't otherwise hear).
Sirius' playlist is considered just the opposite. More focused on top 50 from that genre. This may be better when you're throwing a party and all you want to hear are songs you know, but in the long run I think it gets really annoying.
Technology wise XM has Sirius beat up, down, left, right and sideways. XM has over 1000 land based signal repeaters to fill gaps in coverage. Sirius by comparison has under 200 (~125 I think). XM has satellites in geostationary orbit over the equator and focused on the coasts (this means a consisten direction to point your antenna and better signal on the coasts where the majority of the population is). Sirius has satellites in a figure-8 high atmosphere orbit and will sometime in the next year launch a single geostationary low orbit satellite like XM's. This means that Sirius owners often need to move their antenna depending on time of day or splice together multiple antenna or use larger external antennas to receive the same signal. XM offers real-time weather and traffic services...Sirius will offer those someday.
XM has a truely portable radio (the inno and the helix). Sirius has the stiletto, but the weak repeater network makes the reception very poor.
XM employs more advanced audio processing resulting in what is generally considered higher sound quality, but this is really subjective. XM does offer 'XM HD' which is XM's way of sending 5.1 surround. There are a number of home audio receivers that take advantage of this.
XM's business model is considered more sustainable because they spend less to sign a customer and generate more revenue in places other than basic audio service. Sirius business model is almost entirely dependant on Howard Stern's success.
XM has signed deals with over 50% of the automotive market (by volume, majority of which is Toyota, Honda and GM) and will begin showing up as a standard feature on many vehicles in the next couple years. Sirius has less than 50% and will also be moving towards standardization.
Also, both offer ways to block channels that play content you wouldn't want kids to hear.
So, to answer your question, I prefer XM because I don't care about Howard and the technology and coverage is better.