My car came with XM. I pulled the box and put in a Sirius unit because:

I like talk radio and Sirius has a more complete package with two NPR stations and talk left, talk right, and talk center. They both have similar other offerings of BBC, Fox, etc. but XM does not have a "left" station or NPR.

For music, the selections are pretty similar. Check out their sites and look through the lists. You will also find a roughly equal number of people that think each has a better playlist for each station. Sirius is also nice because you can access all the streams on the internet, which is handy for me at work.

It does really depend on what you want.

The differences are simple.

Sirius has more of the sports channels,
NHL,NBA,NFL.... that is a pretty good ticket!
Sirius aslo has NPR, which a lot of people love.

XM - has NASCAR! and is $3.00 a month less. It also has the largest market share and is expanding rapidly.

Both are commercial free now!

In a year, both will have video!<ul><li><a href="http://public.intranet.crutchfield.com/i-rad1cdixBv/cgi-bin/SuperProdGroup.asp?c=3&amp;s=0&amp;cc=01&amp;g=69000">Satelite radio!</a></li></ul>