Neither is CD Quality. From my experience(I have both), the XM sounds a little weaker than Sirius, but possibly clearer(if that makes sense.. clearer, but more "tin can" type of sound. I prefer the sound of Sirius.
When I first for the Sirius, I had just bought a new car. I had thought it was my speakers that sounded "muddy" but after playing a CD, I realized that it was the receiver. Of course, I'm using the FM Modulator, so by this logic, it can't possibly sound better than FM.
As for Content, Sirius winds hands down.. at least for Rock.
Someone said XM was inconsistant... that's true, but on most stations, they love the same artists. I swear that you'll hear U2, Green Day, Lenny Kravitz, 3 doors down, Peal Jam at all times(I'm listening now, and 3 doors down is on 2 channels, I've often seen the same SONG on two channels at once. Both services repeat, and while XM will throw in a song from BFE, it's often out of my tastes. I hate repetition, and I like hearing music I've never heard, but I want some sort of consistancy.. at least to the genre.. and I find this more on Sirius. Sirus Left of Center is good and consistant, they play Indie/College rock... a similar lineup to XMU, but XMU tends to play emerging hip hop, as well as playing a more "the loft" type of Indie Rock.. it's all over the place. As for Classic Rock, you've got more variety on Sirius. I can't stand hair bands... but they are mostly isolated to "Hair Nation" with some crossover to "buzzsaw". It's contained on XM to Boneyard.. good riddence! XM has Top Tracks and Deep Tracks, Sirius has Classic Vinyl(older classic rock) Classic rewind (more recent, late 70's- about 1990), and the Vault, which is like "Deep Cuts". Of course, I'm sick of Classic Rock, but sometimes the mood strikes... usually while surfing the presets.
And yes, the 80's decade channel plays hits.. but who do you think listens to 80's channels? That channel is for nostolgia... You listen to it FOR the hits. Most people who listen to this type of channel aren't interested in some obscure artist from 1982.
I'll also mention Alt Nation, I feel it gives a good mix of New Music, Indie, and hits of the past decade. It can't be more repetative than Ethel... just more consistant.
I also LIKE the DJ's on Sirius, gives it personality(when they don't talk over music). XM has an automated/mechanical feel.
Side note: Sirius has a serious lack of cool receivers... XM2GO is a great little device(though with it's design flaws) at a reasonable price. Sirius S50 has more functiality with MORE design flaws for 3 times the money. I guess we'll have to wait on that. XM2GO lacks sound quality though.. I don't know about the S50.
XM looks to have a stronger signal, also it's sattelites are stationary so if you're setting at home or a camp site, coverage can go in and out depending on where the satelite is at that given time.
It's all personal preference, I would prefer the Sirius content on XM equipment, but that can't happen... maybe Sirius will come up with better equipment. Oh, and as of now, Sirius doesn't have sports/stock tickers... XM does(and I don't really care.. just wanted to mention it)