Just dove on the Sea Hunter a couple of weeks ago, it was great. Like everybody says, totally nice boat, great crew, etc. They offered a night dive every night (except at Malpelo, no night dives there), and we got to dive Manuelita (the white tips) three times. When I researched it, (and I haven't been on Aggressor), I read lots of not so great trip reports about the Aggressor's general state of repair and level of service. But I also heard a rumor it had been refitted recently, you might want to check on that.
Couple of differences: Sea Hunter uses actual pangas, not RIBs to dive from. Nice, but then again, who really cares THAT much? Also, Aggressor is supposed to be slower, so you have a longer ride to the mainland. But then again, who really cares? It's really really really really long or really really really really really long. (shrug).
What I would try to figure out is flexibility - if people express preferences for particular dive spots, will they try to make it work out? For our trip anyway, Dirty Rock was totally HAPPENING in the early morning, but only so-so by the second dive. Sea Hunter was only moderately accomodating about letting our team dive Dirty Rock as the first dive, rather than the other group, but they did let a guy switch groups who really wanted to do that dive. (They did let us do it on the last day. The issue was they were trying to equalize bottom time between the groups at Alcyone, which was where everybody wanted to go all the time.)
Another thing is the EPIRBS - I heard Aggressor gives them to you, but only the Sea Hunter and Underseahunter have the tracking device. Maybe they got one by now?
Another consideration, check the schedules of all three boats - if all three boats are there, there will be less diving at the great spots, because the boats coordinate and share the spots equally. For example, we had to alternate with the AGgressor on diving Manuelita at night. I would have done it every night otherwise. Luckily, we got to hog Alcyone for like 6 mornings in a row when we were the only boat at the island.
Also, while we were there, we observed some pretty brain-dead dive site selection by the Aggressor crew. The morning dives are the happening great dives; in the afternoon, the viz drops and current and wind come up, so you tend to dive closer in and away from the Bajos. (The Bajos are waaaaaaaaay better than the coastal stuff.) Well, a couple times we observed the Aggressor divers wasting a morning dive on relatively boring coastal spots, when they could have been out on a Bajo. But, who knows, they might have had circumstances we didn't know about dictating that - film crew, extra timid divers, training, who knows.
Also, I have heard over and over that Okeanos is not up to normal Aggressor standards. Just what I heard, but I've heard it over and over in people's trip reports.
Cocos is great. When are you looking at going?