If I'm being asked who I believe more, the person who authored a slide deck selling some kind of surveillance tool to the NSA, or Google's chief architect, their general counsel, and Larry Page, I'm going to go with the Googlers.
Note that the slides have different degrees of redaction across sources. These links should together cover what's available on PRISM.
http://www.guardian.co.uk/world/2013/jun/06/us-tech-giants-n... http://www.washingtonpost.com/wp-srv/special/politics/prism-... http://www.guardian.co.uk/world/2013/jun/08/nsa-prism-server...
Google has motivation to bring clarity to their role in all of this. The NSA would rather keep everything secret. I'd believe Google more too...