The main difference, based on what I read, is that DEC-B, at first 2 months will be sold at a discount. Second, DEC-B can only be used on "some" of the future projects.
Credits doesn't have any discounts and the main difference is that Credits can be used in almost any in-game purchase including rentals, and card purchase. DEC-B cannot be used on those and will only be usable in "some" future projects.