Try them both.
Pricing--equivalent. Both are 12.99/month, both offer family plans, XM's is 6.99 each additional radio up to 4 add'l.
Sirius has Stern, XM has O&A. I care not a whit for 'extreme shock radio', so this is a non-issue.
Sirius has some sports, XM has others. I don't care about sports, non-issue.
Hardware - XM has [imho] better hardware, and the Inno is coming out very soon--portable satellite reciever/mp3 player with 1 GB ["up to 50 hours"] of capacity. Granted, it'll be almost 400 bucks, but that's new tech for you. You can probably get a 2GO or Myfi, portable and records 5 hours of content, for right around 100-120 bucks. And you could probably get a Roady2 or SkyFi for less than 50.
XM just added ten channels, including reviving XMLM for you metalheads [it was previously on the birds, then only on XMRO, then back on the birds]. It's toned down just a bit, I hear more things like Exodus and Suicidal Tendencies and even Black Sabbath instead of just the RARARARRARAAAAA Carcass Cannibal Corpse stuff [though they still play lots of that].
I have *heard* that Sirius's playlists are shallower than XM's. In my 2-year experience with XM, while I have heard repeats, they are very few and far between, and have been heard primarily on:
* the newly-revived XMLM [but maybe they're still shaking up the playlist]
* 153, the Canadian-focused comedy channel
* 20 on 20 [not that I listen to this channel], but that's the point--it's a top 20 channel.
The only thing Sirius has that I'm interested in is Mojo Nixon as a host, but I'd have to give it a listen to see if I'd actually enjoy him.
My favorite channels on XM are:
* 44 Fred - deep classic alternative
* 53 Fungus - punk/ska/industrial
* 41 Boneyard - hard rock/light heavy metal [lots of 'hair bands' on here]
* 27 Cinemagic - movie scores/clips
* 150/151 - XM Comedy and Laugh USA [standup and other comedy - one is uncensored, the other is cleaner]
* I'm giving 42/XMLM a try, but I can only listen to one or two screamcore songs at a time.
Bottom line: I've heard XM has deeper playlists, while Sirius focuses more on stuff you may be more familiar with. Obviously there's personal preference to that, I like hearing songs I haven't heard by bands I have heard. There's pros and cons for both XM and Sirius, but either of them beat the crap out of terrestrial radio. Is it worth paying for radio? If you listen to music, yes. And even if you don't--there's plenty of talk, including news, comedy, golden-age radio, radio plays, etc. When we visited my dad last time, I let him play with my Roady to see if he was interested in taking it--he tuned in to 40's on 4 and Frank's [Sinatra] Place and listened all day.