Been to Palau twice, been to Cocos two times (and Malpelo once).
Will go back to Palau with my daughter in a few years when she's ready for it (about a year away from being certified).
Will go back to Cocos another 5+ times before they pry my fins away from me (or sadly, the sharks disappear).
The difference:
Palau provides consistently good diving with reasonably high bio-diversity, pelagics, and great diving infrastructure (and it's not that hard to get to with options from Japan, Philippines, and the US). It's a bit like "Disneyland Diving"... sign up for what you want to see and they'll likely be able to show you (mantas, sharks, mandarin fish, etc.)
Cocos is primal diving - it's the ocean as you always hoped it would be... one good dive at Alcyone will change your view of diving for the rest of your life.The diving is occasionally hard - currents, thermoclines, and swell can make for tough conditions - then add in the 30+ hour crossing to even get there.
The amount of hyperbole used to describe Cocos makes it seem too good to be true... in reality - that's how good it really is, when it's really good (water temp not too warm, etc).
Best line I've heard when describing Cocos:
- "You will run out of air before you run out of awe"
I knew the depth that Cocos had impacted me personally when I would wake up from dreams of diving with hammerheads months after returning from my first trip there - I thought the 2nd trip might cure me, but nope... just made me want to go back for more.
And as everyone has said - diving Cocos from one of the Undersea Hunter boats is the way to go - they are (by far) the best and safest operation diving Cocos.