I am just a hobby maker, but I've found that vibrating the cup by hand will loosen stubborn powder to allow better dipping. That way, I can dip without creating Mt. Vesuvius.
It really depends on how you are going to attach it and what your bait is molded out of. But....I have used both and I think the plastisol is faster to mold, but the Flex glues in a little getter. Still, both will work for you.