I've been an XM Radio subscriber for over 4 years, I've got one in both of my cars. In one of those cars, I added Sirius, and I flip back and forth every day... I've had lots of time to compare the services directly, so I thought I'd share my thoughts with folks who were shopping for a satellite radio.
I'm sold on satellite radio. I have a 40 minute commute each way to work over a mountain range, and reception for AM/FM radio is mediocre. Regular radio stations have a very limited variety of choices -- I love sports programming and a variety of music programs, but on regular radio I just don't stations that fit what I'm interested in. So when I got a new car 2 years ago, I took it straight to the local car stereo shop and had them install XM.
A couple of things to know up front about satellite radio. First, for XM or Sirius, you need a receiver that is capable of receiving satellite radio. Every major car and home audio company puts satellite radio into many of their tuners now, so you'll have lots of choices (Alpine, Clarion, Pioneer, Sony etc). The costs vary a lot, from a low of about $30 (after rebates) for a small one to a boombox style at around $150 to $1000 or more for an great tuner/receiver. The total cost of the package in my car was about $700, which gave me a nice professional installation of a great sounding car stereo with speakers that plays not only XM stations, but also AM/FM and cds. My uncle opted for a portable, Walkman style XM radio for about $200 that he carries around everywhere. You can also buy small portables that plug into the back of receivers/radios that have AUX inputs, and will play XM through the system. Prices for XM and Sirius receivers are about the same. A lot of new cars come with XM or Sirius as an option, and a few offer a choice...look for it when you buy a new car. XM has a better selection of hardware and systems.
Reception: Overall, it's great...cleaner, clearer, a big step up from traditional radio. According to audiophiles, XM has a better high end signal and better stereo separation than Sirius (my ears are old and I can't tell the difference), though neither have CD quality sound yet. Some car audio systems are designed only work with XM, some only with Sirius, and (more recently) some now work with both -- my car stereo allows me to have both hooked up, and I can switch back and forth. If you're getting it for home, or plan on driving through a lot of tunnels or forests, these services are like Direct TV -- they have to have a clear path to the southern sky to get reception -- if you can't get a corded antenna to reach a window sill or outdoors, you'll have reception problems indoors. Make sure that you have a clear look at the southern sky to point your (very small) radio antenna. XM has a big edge in ground repeaters that rebroadcast the satellite signal in urban areas, so if you live in an urban area, XM might give you better reception.
Next, you pay a monthly subscription fee. Both XM and Sirius charge about $13 a month. You can buy a package for 1-3 years and get the cost down to around $10 a month. Compared to cable TV, the subscription fees for satellite radio are a deal. Prices are likely to move around, so check their websites for up to date pricing.
Programming. For me, sports is important, and both XM and Sirius have some great sports programming. XM has every major league baseball game for every team, plus all the college basketball and football games for the ACC, Big 10 and Pac 10. I'm from Virginia, and this gives me the chance to listen to every football and basketball game that Virginia plays...it's terrific! XM also has all major league baseball games, the NHL, Indy Car and (believe it or not) all the PGA Golf tournaments. If you love golf, it's great to follow along, particularly on the majors.
Sirius has some great sports programming as well -- all the NFL games, NBA games, NASCAR racing, and college football and basketball games from Notre Dame and the Big 10, Pac 10, Big 12, Big East, Ivy League and SEC. And some offbeat sports as well (World Series of Poker on the radio?). They've also got English Premier league soccer, which is great, and CFL eh? . The NFL is where Sirius gets the edge...they not only have all the games, but they carry both teams' radio broadcast...I'm a Bears fan, and I get to listen to the Bears announcers for all the games. Sirius also has a ton of NFL programming during the week, so if you're a big NFL fan, you're always getting talk shows and information about the league.
For me, a big sports fan with a long commute, it's worth having both services -- they each have so much good stuff tied down with exclusives. If you only want one, think carefully about your favorite sports and favorite teams, and pick the service that covers more of what you like. Keep in mind where your radio will be located, and the hours that you're likely to be near the radio. For example, if you commute Mon-Fri and your radio is in the car, it doesn't do you a lot of good to have NFL or college football games on since the games are almost all on weekends.
Yes, there's advertising on satellite radio. Most of the music channels are commercial free, and the amount of "chatter" between songs is really kept to a minimum. Because news, traffic, weather etc. is available on other channels, there are no long breaks in the music. Some of the music channels do have advertising, this occurs when XM or Sirius are just rebroadcasting a regular "open air" radio show. But for the most part, if you want to listen to long sets of music with little interruption, you'll love it. Many other "non-music" channels do carry advertising, such as the (hysterically funny) comedy channels and the talk channels. Sports stations too...both XM and Sirius broadcast ESPN, and the long breaks in the talk format mean ads....lots of them...just as many as on regular radio.
For talk radio, both have a wide variety of good news and talk stations, you can't go wrong. Both offer many of the same channels like CNN, Weather Channel, ABC news, Bloomberg, CNBC etc. They both have traffic and weather stations for major metropolitan areas. XM has more talk stations and probably better news, but Sirius leans more toward edgier programming, and they've got Stern as an exclusive relationship. If you like Howard Stern, he's only on Sirius, and he's been a big plus for them. XM has the morning duo Opie and Anthony, Oprah, as well as Jim Lerher and Bob Edwards. As with sports, there are some definite differences here, figure out what you like to listen to and factor that into your decision.
For music, it's just overwhelming. Both offer such a terrific variety of great music stations, stations that don't stick to tried and true mass appeal formats but stations that play great music that you wouldn't otherwise here...if you love music, you can get introduced to a lot of great new artists, and listen to music without commercials AND with great quality. I love the fact that both XM and Sirius give you the artist and song name for every song they play right on your stereo display panel, so you're never stuck trying to figure out who does that great song you're hearing. Both have about the same number of music stations, Sirius' music programming and DJs are a little edgier.
Lastly, if you're going to invest hundreds of dollars for hardware in the service, you have to consider financial stability of these two companies. Both are publicly traded and from all accounts appear to be professionally managed. Neither service breaks even financially yet, and they have a lot of debt (Sirius went deep in the red to gamble on Howard Stern -- a reported $100 million a year), but they're both adding subscribers and narrowing their losses. The two services would like to merge, but that has to be approved by the Justice Department (antitrust) -- the latest talk suggests that there's at least a 50/50 chance of approval by the end of the year, we'll see. Merger could put a real powerhouse set of programming (imagine all the sports!) on one service, but it could also mean fee hikes.
Bottom line, there are a lot of similarities in the two services, but there are subtle and important differences particurly in sports and news/talk programming -- do your homework by going to the Sirius and XM sites, review the programming guides carefully, then decide. And if you can, buy a system that gives you the flexibility of recieving both services....if you listen to the radio a lot, $12 a month per service isn't that bad, particularly when you consider how much you're paying for cable tv.