Satellite radio in a nutshell: I spent one night earlier this summer driving around and listening to the Washington Nationals close out an exciting win. Where I was driving around, however, was Charleston, W.Va., well outside the broadcast range of the Nationals' Z104/WFED radio network. It was not, however, out of range of XM Satellite Radio, which this season began carrying every game of all 30 Major League Baseball teams and beaming them across North America.

It was terrific to be able to keep up with the team from afar; every win sounded like the World Series on the radio, thanks to the vocal fans.

So, as the final out of this particular game was recorded, the Nats' announcer enthused, "Just listen to that crowd!" It was a cruel taunt. At that precise moment, I drove into one of a handful of dead spots around Charleston where XM service drops out for several seconds.

Hence, the often-simultaneous joy and frustration of satellite radio.

Now, with nearly 6 million subscribers between them, XM Satellite Radio Holdings Inc. and Sirius Satellite Radio Inc. have established a foothold as a competitively priced entertainment option for auto, home and mobile use. Like the cell phone, however, satellite radio is an infant technology compared with its predecessor, which has had more than a century to perfect its delivery system. And, like an infant, it still spits up from time to time.

On the face, Sirius and XM are comparable services: Both have more than 60 channels of commercial-free music covering a broad spectrum of niches, from old-school country to today's hits, from the most experimental jazz to the spaciest New Age, from the raunchiest hip-hop to the kid-friendliest Radio Disney.

Each has channels devoted to music from the decades of the '40s to the '90s; each has bluegrass and standards channels, each has several hip-hop and classical channels and so on. XM has a fun unsigned bands channel that Sirius does not have; Sirius has a groovy, trip-hoppy electronica channel that has no XM equivalent.

Both also have more than 50 channels of news, talk and entertainment and share many of the same third-party providers: Fox News Channel, the BBC, Bloomberg, CNBC and CNN.

And both cost a fair amount of money over time: The receivers for each start at $50 (in some cases, after a mail-in rebate), and each service runs $12.95 a month before any family-plan or pay-in-advance discounts.

But over the course of their short life spans, each service has begun to develop a personality and a direction.

Music fans will find a deeper and better-defined selection of stations on XM.

Sports and talk fans, however, will gravitate toward Sirius, which broadcasts NFL and NBA games. Sirius also has swiped NASCAR from XM, starting in 2007, and will resume NHL games, assuming anyone cares. XM cannot rival this lineup. For live action, it offers only baseball, the PGA Tour, three college conferences and IndyCar racing. Through its carriage of ESPN, it also broadcasts the NBA playoffs but not the regular season.

Of course, Sirius also can claim the sui generis Howard Stern, who probably will be good for 1 million new subscribers on his own after he joins Sirius in January.

By way of disclosure, I have had XM in two successive vehicles since December 2001. Before this comparison, my exposure to Sirius was limited at best, confined largely to rental cars that featured the service. Sirius sent me a loaner boom box in early June and I spent a large part of the month -- consecutive days and otherwise, dropping in at various times of days and late nights, on weekends and weekdays, well over 100 hours -- sampling the service.

In comparing XM and Sirius, I judged two factors: programming and reception.

With the benefit of a year's head start, XM has the better music stations. Each channel has what is known in the business as "stationality": a personality that makes it instantly clear to listeners which channel they are listening to. XM achieves this with a tight but deep selection of music for each channel, custom jingles featuring each station's name and, on a few channels, and sound effects that create radio's "theater of the mind." "Hank's Place," XM's classic country station, sounds like you've stepped into a honky-tonk, right down to the coin dropping into the Rock-Ola.

The allure of satellite radio is its plethora of narrowly formatted stations with little overlap between them. When you want classic alternative such as the Clash, the Smiths and Joy Division, you go to XM channel 44, which is called "Fred." One rarely if ever hears those artists on other XM channels.

By contrast, many of Sirius's channels sound like they're dipping into the same playlist. Flipping back and forth for several hours among Sirius channels "Starlite," "Sirius Love," "Movin' Easy" and "The Bridge" proved one channel indistinguishable from the others. Same with the four heavy-metal channels. If there are subtle distinctions, they evaded me.

While listening to Sirius in a rental car in May, I heard two Sirius channels commit what is, for me, an unforgivable sin for satellite radio: playing the same song at the same time. (Worse still: It was America's "Sister Golden Hair.") This I can get from hoary FM.

Sirius does have some outstanding personalities. I don't know from show tunes but found myself listening to "Broadway's Best" one night for two hours because of the hilarious host, Seth Rudetsky, a Broadway pianist and winner of a Funniest Gay Male contest. Sirius's celebrity deejays include Eminem, who helped program a hip-hop channel, and superstar skateboarder Tony Hawk. (More dudes per hour than any other radio show, guaranteed.) XM counters with Tom Petty and Snoop Dogg.

Fairly, it should be noted that I listened to Sirius at its weakest time of the year in terms of its sports programming. A huge part of Sirius's allure is its carriage of the NFL, NBA and NHL, none of which was currently playing. Sirius in November to January -- when all three leagues are in season -- must be nirvana for sports fans.

As for service, each relies on a different set of satellites. Sirius's three spacecraft ride a set of looping orbits that bring one at a time across the United States, while XM's two spacecraft sit in a more distant orbit that keeps them in fixed spots over the country.

Here's the problem: Satellite signals, unlike sturdier AM and FM signals, travel like sunlight and can't pass through obstructions such as buildings or dense foliage.

Sirius says the changing paths of its satellites minimize the areas shadowed by any ground-bound obstacles. XM compensates for its dead zones with hundreds of land-based repeaters that pick up the satellite signals and beam them around larger cities.

In places such as Washington, XM service is nearly flawless. In smaller cities such as Charleston, which has fewer repeaters, you get dropouts when driving along a southern ridgeline and between buildings downtown. Sirius's service in those spots is better but still has hiccups. Also, because Sirius's satellites move, I had to relocate the Sirius antenna from one window to another to find the signal over several hours of listening one afternoon.

As AM and FM convert to digital broadcasts and add additional channels to their signals, over-the-air radio probably will offer new competition for satellite. Digital radio will also allow AM and FM to go beyond their current limited ability to broadcast such text info as a station's call letters or the title and artist of the current track. (XM's baseball channels, for example, not only display the score but the situation -- "BOT 8TH 1 OUT" for bottom of the eighth inning and one out -- to bring listeners instantly up to date when they tune in.)

But local terrestrial radio stations will never be able to duplicate the wide variety of music, sports and talk choices and national reach of XM and Sirius, making both satellite radio services, even with the occasional signal glitches and sometimes-inconsistent quality of programming, far superior to their over-the-air cousins.

XM chief executive Hugh Panero, left, and Bob DuPuy, chief executive of Major League Baseball, announce a broadcasting agreement.Sirius Satellite Radio deejay Dusty Street making her first live broadcast on July 2. Sirius has a number of deejay personalities and sports programming.