I just did that research. But I haven't been yet. Hafta wait till April.
First, keep in mind that all the 2006 Cocos/Malpelo charters are full (with the Sea Hunter anyway. Underseahunter doesn't go to Malpelo, only Cocos. And the Okeanos Aggressor gets consistently much lower reviews than those other two boats - check out the trip reports on Diver2Diver and Undercurrents.).
Best season for Hammerheads is July-Oct or something, in Cocos, but they report seeing them all times of year, although not as consistently or in as great of numbers.
Easier to get to? Depends how you define it.
Cocos is this: Nonstop flight (I paid $250) to Costa Rica from Los Angeles. They pick you up in a bus and drive you 3 hours to teh coast, wehre you board your liveaboard. Then you suffer through a 36 hour trip to the dive site, but, remember, you're already installed in your liveaboard.
As I recall, and somebody who is recently back from Galapagos please chime in, you have to first fly to Quito, then you have to overnight and catch another plane out to the Galapagos, where you have to catch a bus and then a ferry (?) to the boat. More airlines, puddle jumpers, and baggage restrictions. Bigger PITA factor.
Doorstep to stateroom, I'd say Cocos is easier. Maybe not if you get very seasick though. But that's why they made the patch!
As far as big animals, and assuming you go in the right season, they both have tons. I've read so many reports they tend to blend together, but they both have lots of whalesharks (maybe Galapagos more?), Hammers, Galapagos sharks, silkies, white tips, rays, and the occasional humpback.
If you go to the Sea Hunter website,
www.underseahunter.com, they have "trip reports", where they give an extremely brief blurb on what each charter saw.
Most of the Galapagos charters have at least a few land excursions, so if you have an interest in land stuff like boobys and penguins and iguanas and sally lightfoot crabs etc., then go there. I chose Cocos because I didn't want that stuff to cut into my dive time! ;-)
You have to book waaaay in advance though. Although I do know that Hollywoodivers has a trip going next year that apparently has space still.
Have Fun!
Taxgeek