We did the Sun Dancer II in Belize in May last year and loved it. It was a very nice boat, great crew, and great food. We loved it so much that we booked the Wind Dancer in Tobago while we were still on board and just got back from there. The Sun Dancer does try to do the Blue Hole, but we were unable to get in while we were there as was another group from our area that was down in July, 2005. We were a little disappointed, but to get to the Blue Hole, you have to give up 1/2 day of diving to get there and the conditions have to be just right for the big boats to get into it. When we were there, the conditions were questionable at the Blue Hole and we were having too many great dives to give any up on a small chance we might make the Blue Hole. We don't have any regrets about our decision, although I would have loved to have seen it from the air.
The Sun Dancer II and the Aggressor share the same dock, so we saw the Aggressor also. Both boats are very similar, but in my opinion the Sun Dancer looked alot nicer (I didn't get to go aboard the Aggressor). They also include your dinner on Friday night, which you have to go into town to get on the Aggressor. I think either would be a great trip though, and definitely the only way to dive Belize.
Have you done Utila before? We've just locked in our airfare for a trip there in September and it will be our first time there. Just two of us this time to check it out before we bring the whole group.