Opie & Anthony, even with a forthcoming assist from Oprah Winfrey, are proving no match for the marketing juggernaut that is Howard Stern.

Sirius Satellite Radio, home to Stern, said Thursday that it added 600,460 new subscribers during the second quarter, a 64 percent increase over the same period a year ago. Rival XM Satellite Radio added 398,000 users, representing a 38 percent drop in its year-over-year growth.

XM chief executive Hugh Panero said subscriber additions were muted by "product availability and overall softness in the retail channel." He expressed confidence, though, that Winfrey's addition--by way of the Oprah & Friends channel due in September--should reinvigorate growth.

While Sirius beat analyst estimates of adding about 555,000 subs in the quarter, XM missed estimates of about 408,000 subs. XM, though, remains the industry leader with 6.89 million subscribers at the end of the second quarter, compared with 4.7 million at Sirius.

Sirius has been gaining rapidly on rival XM since Stern began hyping Sirius more than a year ago. XM responded to the Stern effect by making its often raunchy show starring Opie & Anthony, previously a premium product, free to all XM subscribers and, in an industry first, simulcasting the less-offending portion of the show over the airwaves via CBS Radio.

Sirius has forecast that it will end the year with 6.2 million subscribers, and XM has indicated it will end the year with 8.5 million subs.

Investors, who have been chipping away at shares of Sirius and XM all year, seemed unimpressed with XM again Thursday, bidding shares down 2.6 percent to $14.11. Shares of Sirius advanced 1.1 percent to $4.53.

Shares of XM are off 48.3 percent so far this year; Sirius shares are down 32.4 percent.

"On a net add basis, this is the third quarter in a row that Sirius has added more than 55 percent of the industry subs," said Bear Stearns analyst Robert Peck, who predicts a good third quarter for Sirius when it benefits from NFL-related promotions. Sirius is the exclusive satellite-radio home of the NFL, while XM is the exclusive sat-radio home for Major League Baseball.

Sanford Bernstein analyst Craig Moffett seemed more bullish on both companies, writing in a research note Thursday: "Broadly speaking, in any dupoply subscription business, there is a tremendously strong pull toward 50-50 market share."

While recommending that clients buy shares of both companies, Moffett said he favors XM because of "the relative valuation gap" in shares. Sirius, the smaller in terms of sub count, sports a market capitalization of $6.36 billion to XM's $3.65 billion.