It's a safe bet that Oprah Winfrey and Howard Stern travel in different circles. But in a move that has the broadcast business buzzing, they may become professional colleagues.

Earlier this week, America's two satellite radio companies, XM, which carries Winfrey's radio show, and Sirius, Stern's radio home, announced plans to merge.

While the merger must be approved by the Federal Communications Commission, critics say an XM-Sirius merger would create a monopoly and offer listeners' fewer choices.

"Historically, choice is good for consumers," said Dennis Wharton, executive vice-president of the National Association of Broadcasters, which immediately issued a press release opposing the merger. "It's good for keeping prices down, and it's good for keeping diversity of format and diversity of programming."

For $12.95 a month, Sirius offers subscribers 198 stations. XM offers 247 for the same price. Of those, Sirius carries 69 music stations and XM offers 116. Both companies say the range of music available to listeners would actually increase as a result of their proposed $13 billion merger.

But some subscribers worry that the consolidated companies would stick them with fewer options within musical genres, especially those like jazz and classical music that get little airplay even on satellite radio. Both satellite services currently offer a total of six classical stations.

"If one station is playing Beethoven and I want to hear Stravinsky," said XM subscriber Peter R. Webster, the John Beattie Professor of Music Education and Technology at the Northwestern University School of Music, "I can hear Stravinsky on another channel."

FCC approval of the merger, which would combine XM's 8 million subscribers with Sirius' 6 million, is not guaranteed. Critics of the merger point to the FCC's decision four years ago not to approve the merger of the two satellite television broadcasters, EchoStar and DirecTV.

"The regulators looked at the deal and ultimately disapproved it," Wharton said. "The parallels are striking. ...If past is prologue, then I think that the hurdles for getting this approved are pretty high."

If the FCC defines satellite radio as a self-contained niche, a merged XM and Sirius would clearly become a monopoly. If, however, they are considered to be in direct competition with other outlets, like Internet radio, the merged company might be able to avoid the monopoly label.