I’ve had SIRIUS for several months and I love it. I recently went on a road trip from Washington to California, and it was great to be able to hear the same music and talk shows for 1000+ miles.
I also got to check out XM for a few hours in my mom’s new car. I immediately noticed two things:
XM has more music stations and fewer talk/entertainment. I believe they also have more channels overall.
SIRIUS has noticably better sound on their music stations - about CD quality. XM sounds about as good as FM radio.
Other differences I’ve picked up from satellite radio forums:
XM has more ground repeaters, so if you live in a big city, XM is more likely to give you a good signal where you can’t see the sky (under a bridge or in a parking garage). SIRIUS has one more satellite than XM and better orbits, so when you can see the sky, you’re more likely to get a good signal from it.
SIRIUS is more hits oriented; XM has deeper playlists. I only listened to XM for a couple hours, so I can’t say if they have more or less repetition than SIRIUS.
SIRIUS’s music channels are all uncensored; I believe XM has some censored and some uncensored channels. I still catch SIRIUS playing the “radio edit” version of a song once in a while, though.
You can listen to SIRIUS over the internet, at reduced quality. Subscribers can listen as much as they want; guests can only listen for 20 minutes. XM has no internet streaming.
SIRIUS makes a big deal about its coverage of the NFL, NBA, and NHL. I’m not sure how much live sports coverage XM has in comparison.
Two of the big differences between XM and SIRIUS are now gone: XM has no commercials on their music stations anymore, and SIRIUS is running a promotion that makes it about the same price as XM (if you pay for 15 months in advance).
The equipment is mostly the same in terms of features and price. The channels are mostly equivalent, though SIRIUS has more channels in some genres than XM, and vice versa. XM has the new Air America talk network, but SIRIUS has had its own liberal talk station for ages.