Does anyone know the cost difference between the two? I would really like to use the blue board with a skim coat of plaster.
Probably not the answer you want, but it really depends where you are. If you are in Mass., where there are a lot of plasterers, the price difference will be small.
I am in North Carolina.
You should call some drywall companies in your area and ask if they offer a "gypsum veneer plaster system" (commonly called "blueboard"). The plaster veneer is only about 3/32" thick so it is not a traditional coat a plastereer would apply to lath. There is no reason for it to cost more if the company knows how to install it.
For a smooth wall/ceiling finish, If you go with a level 5 drywall or plaster over blueboard, you are going to want to be real careful about who you get to do this. Take your time in seeking out quality,reputable finishers. If plaster over blueboard is available in your area, i would go with that even if it is more dinero, because a level 5 finish can't compare to a decent plaster finish imho. Get real world estimates from outfits in your area.