So I'm sitting here thinking that a measure of how popular a product is would be a measure of how many people would go to illegal lengths to acquire it. In the RPG world, one measure would be torrent downloads of books.
A quick search on Google produced two links which purported to have basically all that Paizo has produced, and all that WOTC has produced, in two separate downloadable packages. I used links from the same torrent site, as well as made sure they were recently tracked/updated at the same time, in an attempt at making this an accurate comparison. (I almost provided the links used, but didn't want to make it easy or seem to encourage illegal downloading. This was purely a shit-stirring intellectual exercise).
The scoring system? Basically look at how many leeches, meaning people who want the stuff, are online with a torrent client trying to download it.
The score: Paizo 169, WOTC 13. Meaning that only 13 people are trying to download all of WOTC's 4e stuff, as opposed to 169 trying to download all of Paizo's.
Paizo wins again!
Disclaimer: I don't advise, advocate,condone, or recommend that anyone actually go out and download this stuff. Give your money to the people making it. Buy it. It's a tough economy, the designers and publishers need the cash. Plus, it's illegal. Plus, they are tracking it big-time now. There is a strong likelihood that you will get caught and get your ass thrown in jail. Plus, don't be an asshole. Before the accusations start coming, for those of you not in the know on Torrents, its not illegal to do a search on Google to see what's available, as far as I know. It would be illegal to download it.