I just got the Sirius Sportster as an early Christmas present just last weekend (the wife was tired of hearing me complain about the lame broadcast radio here! hehe)
I gotta tell ya, Sirius ROCKS. I know a few people with XM & they are equally passionate about it too.
The necessary equipment is the same price. It comes down to which programming you want. Music-wise, they're both pretty much a dead heat. 60+ channels of music, all broken down into genre & sub-genre. Both services display artist & song name as it is playing. Both are commercial free music too. Neither are regulated by the FCC so they can say whatever the <beep> they want

That leaves the talk, entertainment, & sports programming.
Some XM benefits:
* NASCAR
* MLB
* Several major college conferences
* Coast to Coast (another hot CH thread topic)
* Less expensive than Sirius if you go month to month payment
Some XM drawbacks:
* Clear Channel owns a significant stake in company (& supplies or at the very least has influence in the content)
* There are premium channels that cost extra $ per mth
Sirius benefits:
* NFL (including seeing what teams are playing with score & what quarter in real time on the display!)
* NHL (or is that a drawback???)
* NBA (with scores)
* Several major college conferences (but different ones than XM)
* No premium channels--you get all channels with subscription
* Howard Stern is coming in January 2006 (OK, might be a drawback to some! but not me

)
* For $500, you can buy a LIFETIME (read: no expiration) service contract.
Sirius drawbacks:
* None that I can think of...
Both also offer attractive programs if you pay quarterly, bi-annual, annual, & multiyear agreements. Discounts are available if you have multiple tuners too (like one in your car & one in the wife's).
HTH!