Being a subscriber of both XM Satellite Radio and Sirius Satellite Radio, I can now give you some information about both. Both services have its strengths and weaknesses.
First off the price for both services is identical, $12.95 per month. XM lets you pay per month via a debit or credit card. Sirius only lets you pay quarterly (every 3 months) when you sign up online. There is no option to pay monthly online. If so, its hidden.
If you're going to be using satellite radio just in your car, you shouldn't have to worry too much about locking onto the satellite signal. Although since XM's satellite is stationary in the sky and lower sometimes it will be hard to keep a signal while driving through areas of high buildings and a lot of tall trees. Sirius has a few satellites that move around in the sky, plus they are higher up so its a little harder to lose the signal while driving.
Now if you're going to be using the satellite radio just indoors, you'll have a lot better luck with XM. You can basically mount your antenna in a window facing southward (in NY anyways) and the signal will go through windows and even some walls. This isn't the case with Sirius. With Sirius you're more than likely going to have to mount your antenna outside your window or just by your window to get a strong enough signal. Since XM's satellite stays in the same location in the sky, you can just place your antenna and forget it and you'll probably always get a good signal. Not the case with Sirius. With Sirius you'll notice that there will be times where you'll lose the signal completely. I'd say about an hour or 2 a day. There's really no way around this unless you go and physically move your antenna again to try to find the satellite in the sky. Yes, the Sirius satellites do move around in the sky. Some people have overcome this by mounting their antennas directly on their rooftop. This choice is not always possible for some people.
As far as music goes, you'll notice Sirius is probably more similar to FM. The reason I say this is because you'll hear a ton of hits. Sirius focuses more on just playing singles from albums. Sirius is also really bad about expanding their music playlist. After a few weeks you'll notice the same songs playing over and over. XM is a bit better. They seem to have wider playlists on most channels. Also, they open up their playlists to include lesser known songs, and the other NON-single songs on albums. So you'll usually always get a nice variety of music.
Now if you're into talk radio, Sirius has Howard Stern exclusively, while XM has Opie and Anthony. You can listen to these guys for free with your subscription. There is no longer any premium cost. Being a fan of both, I am forced to subscribe to both services. Sirius also has Playboy Radio for free while XM doesn't carry it at all anymore.
Ultimately, its your own choice, but above are some facts to base your decision on. You can also check out each service's website for even more information: