I'd say the FSF has a better reputation than most organisations in this space, and certainly a more established one than Purism does. Here in this thread there are a lot of people criticising them for being "too dogmatic" or for applying their rules too inflexibly, which IMO is proof that they take a firmly principled stance - which is exactly what's needed to validate something like this. A group that's willing to bend would soon end up being meaningless.