The accuracy of either guild progress tool will be down to guilds updating their info. Personally I prefer the guildprogress.com tool as it is a tool I am used to having used it many years in eq2, and the verifying of kills isnt as protracted/fiddly as new tool.
So imo for accuracy everyone should use either both tools, or pick one which seems unlikely.
I dunno, raidranks looks better and has no ads.
Even the FAQ link on guildprogress takes me to an ad but no FAQ's.
But I guess it doesn't hurt to have two sites in case people don't know about one or the other.