Good morning Lyndatu
I guess it comes down to preference over balloon or black Mollies. Personally, I love the black mollies. I think they look very striking in a community tank. I would love to have them myself, but the PH is too low for them here. I am always drawn to that tank at the LFS.
Some folks like the balloon shape in tropical fish, I personally do not so would never go with Balloon mollies for that reason. Besides... are the balloon mollies genetically altered to look that way?
gooooooooooooo with Black Mollies..... ( subliminal message lol...)

~ kate