Well, I'm no Stern expert, but he has name dropped Sirius a lot on his show apparently, said nothing of XM yet. He could choose to be really powerful by attempting to singlehandedly move Sirius from underdog, which is it to XM, to the market leader. He would effectively be more powerful for the company than the CEO within the company who would pay him.
It's an interesting situation. At that age and with that many years of experience when money doesnt really matter as much as flexing your power and ego, I think he would almost want to do it just to spite the FCC and clearchannel.
One thing is clear though, if he does go to Sirius, the stock will jump, and will cement Sirius and the brand leader, especially with their NFL deals and other sports deals that XM doesnt have.