(MONEY Magazine) – Satellite radio is the latest "buy what you know" stock phenomenon. As with Krispy Kreme in 2001 and the iPod-era Apple, shares in XM Satellite Radio (XMSR) and Sirius (SIRI) are zooming as more consumers—and investors—discover their product.

In 2004, subscriptions to the ad-free, 100-plus-channel services jumped from 1 million to 4.2 million. Sirius grabbed headlines by signing shock-jock Howard Stern to a $500 million deal, and XM has a new portable receiver aimed squarely at the iPod market. The buzz has sent XM from $3 a share just two years ago to $40; Sirius has gone from 54¢ to nearly $8. And there's not a dime of profit between them. "In the cycle of stock market emotions, satellite radio is somewhere between thrill and euphoria," analyst Kit Spring of Stifel Nicolaus notes in a recent report.

The total market value of XM is $8.2 billion, which works out to investors paying $2,500 for each subscriber, most of whom pay $9.99 a month. (In other words, each will give the company back $2,500 in, oh, about 20 years.) Sirius, which has fewer listeners but gets $12.95 a month, is valued at $10,000 a subscriber. The two companies together are worth almost as much as radio giant Clear Channel Communications (CCU), which has more than 30 times their combined sales.

But satellite is different, right? To some extent. Analysts expect tenfold growth in subscribers over the next decade, and the FCC has granted XM and Sirius the only two licenses for the next 90 or so years. And since it doesn't really cost more to broadcast to 50 million than it does to 1 million, analysts figure that 80% of the firms' incremental revenue could go to profits once they reach the break-even point.

So a small position in one of these stocks isn't nutty for a very aggressive investor. But XM is a far more plausible bet than Sirius. Sirius' high-profile (and expensive) deal with Stern has led investors to push it to a higher total market value than XM, despite far lower sales and a smaller subscriber base. That's putting a lot of faith in the King of All Media.

NOTES: As of Dec. 17. [1] Actual 2003 earnings, plus estimated 2004. SOURCE: Thomson/Baseline.