Wow, do we get this question a lot! There are many opinions, here's mine honed finely over time.
I have both. Originally bought XM. When I got my Magnum R/T, I got Sirius included. But let the subscription run out after the first year and went back to XM. There are very minor differences in the major music categories. You'll choose because of the few stations you listen to most often.
Don't forget to consider the receiver. You'll have to do some research there as well. Whether to go portable or installed in your car. I have a portable XM unit in my car that I can transfer to the house and listen thru my home system. I also have Sirius built into my car stereo, but as I mentioned, after the year I dropped Sirius and went back to XM.
Both have similar products and channels at similar pricing. And, again right now, both are about $13/mo. The available channels, with certain notable exceptions are very similar. For instance Howard Stern fans prefer Sirius.
You can listen to both online (below) and make your decision. Not all stations are on line but there's a good sampling.
Be sure to take special note of the sports channels. They are quite different, so if you're a sports fan, this will be important to you.
I didn't like the Sirius Comedy Channel, preferred XM's. So that's why I went back to XM and have been happy.
BUT everything's going to change pretty soon. The two will probably merge into one company and you'll be able to pick between the two full services, including sports at $13. Or select your favorite 50 channels from both and get that at a special rate of $6.99. There will also be other deals available if the merger is approved. It is currently under review by the FCC, FTC and DoJ.
My advice is to check the available receivers, be sure to shop around at Costco, Sam's etc and also the big electronic outlets like Circuit City and Best Buy. Then check the channel titles and descriptions, sample both services online and pick the one that you like best.
In any case, go ahead and grab one, if the merger goes through it will benefit all.
-a guy named duh