Barry Greenhill did not come to Circuit City in Tysons Corner for a $188 15-inch, LCD flat-screen television, or a $200 laptop computer. He came for one thing: Howard Stern.

Greenhill has been a Stern fan since the radio shock jock worked the morning drive shift at DC-101 in the early 1980s. With Stern set to jump from Infinity Broadcasting to Sirius Satellite Radio on Jan. 9, Greenhill chose to follow. Last month, he purchased a Sirius radio for his car and began subscribing to its programming. Friday he was back to buy a docking station so he won't miss any of Stern's expletive-filled rants while at his federal government job.

"I guess I'll have to use headphones," he said.

New York-based Sirius and D.C.-based XM Satellite Radio are competing for customers like Greenhill this holiday shopping season. Each is expected to sign up a million subscribers during the last three months of 2005, analysts said.

This year, the four weeks after Thanksgiving are especially critical for both companies. It will be the first major test of how many subscribers Stern can attract for Sirius and whether those numbers will justify Stern's five-year, $500 million deal. XM has promised Wall Street that it will exceed 6 million subscribers by year-end, setting the stage next year for the company to turn a profit. Neither company has yet done so.

Both companies charge subscribers $12.95 a month. XM offers about 150 channels of music, news, talk and entertainment. Sirius offers about 120 channels. They get customers from two major sources: automakers and retail stores.

XM, with 5 million subscribers to Sirius's 2 million, has a larger share of the automobile market, with deals to install XM receivers with manufacturers of nearly 60 percent of all U.S. cars, trucks and sport-utility vehicles. As of the third quarter, XM had a bigger share of the retail market, too.

But with Stern's debut on Sirius a little over a month away, Sirius is hoping to edge past XM, at least in the aisles of stores such as Circuit City, Best Buy and Wal-Mart.

Analysts say hardware may play a larger role in future retail battles. Sirius recently came out with its first wearable device, the S50, which can play downloaded digital music and several hours of recorded Sirius programming. Next year, XM will introduce its first portable satellite radio receivers combined with digital music players.

But now, at least, the competition is mainly over content. The theme of Sirius's holiday marketing campaign is "Huge gift. Little box."

"The gift is really the programming and the content Sirius delivers versus just going out and buying a gadget," said Jim Collins, a Sirius spokesman.

Content is at the center of XM's "Listen Large" marketing campaign, which features an all-star cast of famous names, including comedian Ellen DeGeneres, New York Yankees shortstop Derek Jeter, country singer Martina McBride and rapper Snoop Dogg.

What they have in common, besides shows on XM, is their un-Sternlike qualities.

Even Snoop Dogg's "Gin and Juice" days seem far enough behind him for former Chrysler Corp. chairman Lee A. Iacocca to share a golf cart with him in a recent Chrysler ad.

In a television spot for XM now running nationwide, Snoop Dogg is seen rambling through XM's studios in Northeast D.C. in search of some misplaced bling, and running into DeGeneres, Jeter, McBride and David Bowie along the way.

"Our campaign this season is to make sure consumers know with XM they get the broadest range of choices and not programming tailored to just one segment of the population," spokesman Chance Patterson said, referring to Sirius's half-billion-dollar bet on Stern.

Both companies have prepared for an intense retail battle and are engaged in one-upmanship. Both offer $50 rebates on hardware. Sirius is also offering a $50 gift card that can be used to cover the subscription fee; XM's gift card is worth $55.

XM saved up revenue from an increase in its basic subscription fee this year to spend on promotions during the holiday shopping season, while Sirius set up a Howard Stern Web site featuring short films as part of its "viral" marketing campaign.

"No one will not know that Howard is coming to Sirius," Sirius chief executive Mel Karmazin said several weeks ago during a conference call with analysts.

The ad blitz will extend into the first few months of next year, with Stern taking a more prominent marketing role after his contract with Infinity ends Dec. 31. Until then, Sirius is limited in how it can use Stern and his image in its promotions. A passport-photo-size Stern appeared on a Sirius kiosk at the Tysons West Circuit City, the same amount of real estate given to fellow Sirius star Martha Stewart.

In the meantime, Sirius has dreamed up interesting proxies for the foul-mouthed radio host, including an ad running in movie theaters that features stars in a darkened sky forming a constellation not typically seen outside of a pair of boxer shorts. The ad's text consists of three phrases: "Coming in January. Howard Stern. Only on Sirius."

Derek Jeter of the New York Yankees appears in a television commercial for XM Satellite Radio.