You can go to my website and see a trip report, gallery, and slideshow (with video) of our Undersea Hunter trip last Sept.  We had such a great time we are doing a Malpelo/Cocos trip in 08.  We are Aggressor fans, have dove them at several locations and always enjoyed ourselves.  We went with the Undersea Hunter for 3 basic reasons:
1. They take fewer divers.  This does play some role as the hammerheads are shy.  The fewer bubbles, commotion the better luck you have of them coming in close.
2. I like the pangas that the Undersea Hunter runs.  When we were there the water was fairly rough, swells and +3ft chop.  It looked like the Aggressor zodiacs where having a rough time of it and moving 1/2 the speed our pangas were moving.
3. I do every dive possible.  Aggressor "said" they need minimum of 3 people to do night dives so there maybe 1-3 nights they don't go out.  Undersea Hunter, 1 person wants to go, you go. What the office says and what happens on the boat may be very different.  But trust me in Cocos you don't want to miss a dive.