Jester David
Hero
Do you have the exact numbers with regards to magnitude?
Just downloading the rules doesn't really count as playtesting. Paizo had over 100,00 contributions to the Pathfinder rules so the title is a subjective one. We don't necessarily know who's was bigger but I think Pathfinder was the biggest.
Paizo has reported close to 45,000 participants of their playtest. WotC reported 200,000 playtesters, over four times as many. The Pathfinder number is also downloads, which might include repeated downloads by the same people. The WotC number is sign-ups for the program, so the number of downloads would be much higher.
Pathfinder had 100,000 posts on their message boards, but a number of replies were likely made by the same people. It's unlikely every playtester even posted.