Historically the Dancers tended to have more service and be a bit "plusher" - fluffy robes, turndown service with chocolates on the pillows, beverages left at your door in the am, neckrubs and hot towels after dives. Probably not $500 worth though, may or may not be what you want, and I don't know if it is true in this case anyway. Along the same lines, Dancers have also tended to have dinner served vs buffets. Again, I don't know in this case - and I have usually preferred the food and buffets on Aggressors anyway. (Even though I'm usually a sitdown rather than buffet person, buffets just seem to work better on a liveaboard with that size group as far as food choices.)
Also, the Aggressor boats usually have a single over double bunk arrangement, while the Dancer boats have usually had beds. I find the bunks a bit claustrophobic, though the top bunk is a handy place to put lots of things if it's not being used. But not true in this case, both boats are bunk style, as the Dancer was recently an Aggressor boat and they are nearly identical. I've lost track as boats have moved around over the years, but I think these 2 were considered sister ships.
It would be interesting to ask the company what accounts for the price difference and see what they say.
BTW, common misconception that Aggressor owns Dancer, they do not. Both Aggressor and Dancer were bought by a third company.