Depending on how broadly you define the competition, Sirius XM's (Nasdaq: SIRI) competitors include terrestrial radio, Apple, and all sorts of Pandora-type online companies. I recently asked Motley Fool analyst Rick Munarriz about the single biggest threat to Sirius XM.

Rick Munarriz: The biggest threat right now would have to be mobile connectivity. These days, all the automakers are trying to get the fanciest dashboard gadgets out there, and a lot [of] them, like Ford (NYSE: F) Sync, have ways for you to actually tap into the Internet from your car from your very smartphone. You can tune into Pandora music or CBS's through your smartphone or if you bring a USB card or broadband modem; you'd just plug it right into your car and connect to any Internet radio station that's out there.

Sirius XM was an easy sell over terrestrial radio because commercial radio is full of commercials. It's full of ads, it's limited in scope, playlists tend to be repetitive on the music stations, and you get tired of the local celebrities on regular radio. Sirius XM was a serious step up in the quality and quantity of available content. What connectivity does, what being online in your car does, it opens up the spectrum even wider. And sure, there are going to [be] problems with streaming speeds and access as you drive around the country, but for most people who are paying $15 a month for their Sirius XM subscription, if they're already paying $30 or $60 for a broadband modem card, or a mobile plan with unlimited data, they're going to go ahead and milk that for all it's worth. And that includes enjoying what they can in their cars through these systems.

And that's the real threat for Sirius XM -- not so much that the quality of its products is going to deteriorate, but that the availability of stuff that's already paid for by the drivers is going to eat into the market share of the time that somebody's ears can spend listening to content.

Greer: So if, in the next three to five years, Internet radio is ubiquitous in cars, and I can listen to Internet radio in my car, what does Sirius XM look like then, and can it compete?

Munarriz: Sirius XM can compete in the future because it has the ability to pay for content that Internet radio will never be able to. There will never be an Internet radio station on its own that will be able to pay for Howard Stern through ads, through an ad-supported model. There'll never be someone who'll be able to talk Oprah Winfrey or Martha Stewart or Rosie O'Donnell into any kind of content deal where they get paid a tiny little sliver of online ads for their time. So I think the exclusive content ---the major leagues, the NFL and Major League Baseball --- is not going to be streamed unless they're controlling the streams and making a good chunk of it. Sirius XM has the ability, knowing that they have all this revenue coming in from subscribers, to pay up for content. Whether or not that means signing Howard Stern beyond next year, Sirius XM is going to have some star talent to differentiate itself from the Internet streams that are available from some guy spinning old-school rap out of his basement.