I would recommend that you do the twilight dive, just before the night dive (some ops offer that). You may get to see mantas during the twilight dive as well (we did).
I would also recommend that you do it twice (that's what we did). The first dive was overwhelming (in a good way), and since we knew what to expect, the second dive was even more pleasurable. One other advantage of doing two dives, is that you could be right in the middle of the action on the first and a little removed on the second to get different perspectives.
Don't bother with pictures, they usually have a videographer on the dives and splurge for the DVD.
Finally, you may get a visit from crazy frank, the resident and very friendly moray eel.
Have fun, it is an unforgettable experience.