Is it time to trade in shares of Sirius XM Radio (NASDAQ:SIRI) for Liberty Capital (NASDAQ:LCAPA)? The Wall Street Journal's "Heard on the Street" column yesterday seems to think so.

It explores the springtime investment that Liberty made when Sirius XM was on the ropes. The satellite radio operator chose Liberty's lifeline over EchoStar's (NASDAQ:SATS) offer, and that decision has paid off nicely for the victorious bidder. In exchange for lending $530 million to Sirius XM at a chunky 15% interest rate, Liberty also received preferred stock equivalent to a 40% stake in the sultan of satellite radio.

At current prices, that 40% convertible stake is worth roughly $900 million. When you tack on the $530 million that Sirius will eventually have to repay, you pretty much hit Liberty Capital's current $1.4 billion market cap.

"The Sirius interest is worth roughly $15 for each Liberty Capital share," the column concludes. "And that doesn't include other assets reflected by Liberty Capital, including the Atlanta Braves baseball team and stock in Time Warner (NYSE:TWX)."

The column leans on a valuation by Hudson Square Research, which pegged a net asset value of $23.32 a share on Liberty Capital's tracking stock.

Now, I see your wheels turning. If Liberty Capital's market cap is essentially the value of its stake in Sirius XM, why not just buy Liberty? It will appreciate if Sirius XM's stock continues to rise, but there's a diversified cushion -- and even a steady trickle of Sirius XM debt interest payments -- if it doesn't.

The column even speculates that Liberty Capital may want to consider spinning out its stake in Sirius as a separate company.

Don't hold your breath. Liberty Media is already a tangle of tracking-stock spaghetti strands. There's Liberty Entertainment (NASDAQ:LMDIA), Liberty Interactive (NASDAQ:LINTA), and Liberty Capital, along with matching B-share configurations. If anything, Liberty needs to simplify its structure,

It would also be a mistake to assume that Liberty Capital's stock will rise in lockstep with Sirius XM. There's more to Liberty Capital than just its $1.4 billion market capitalization.

As of the end of March, Liberty Capital held $3.8 billion in non-strategic public holdings, $2.8 billion in cash, and $6.6 billion in debt. In other words, there are way too many moving parts to assume that Sirius XM is anything more than a supporting player here. And continued appreciation of Sirius XM will naturally have a favorable impact on Liberty Capital's 40% stake, but it doesn't increase the value of the $530 million loan.

In other words, going long on Liberty Capital, and shorting Sirius XM, isn't the no-brainer layup it may seem to be. The company certainly offers a compelling way for risk-averse investors to play the improving cash flows of Sirius XM, along with a basket of other media assets. Unfortunately, it's not a perfect substitute for the real thing.

Other ways to slice and dice satellite radio fandom: