I don't have a position in either P or SIRI. I use both services. I would dump SIRI before P, just sayin'. P needs to charge a profitable price per user, who cares how many drop it. They will make money off the ones that are left, something they can't do now.
Pandora has limited content---as in NO ORIGINAL content, which puts them at a decided disadvantage to Sirius. Pandora is really just competing with many other free music services---and the field they are competing with is only growing. Many people would immediately drop Pandora for another dime-a-dozen internet music service if Pandora decided to charge for all their non-original content.