Both are independent modules and has a different purpose.
X-Fee/Discount Pro is an `Order total` module that deals with shopping cart products and their values like cart weight, cart quantity, etc. This fee/discount will appear on the `Order Total Section`. You can create a discount/fee based on the various parameters with it. For example, Payment processing fee, any surcharge, etc. Also, you can create Order discounts based on the various cart parameters.
On the contrary, `X-Discount/Special Price` is intended to creating a product special price and quantity discount. It works exactly the same as the OC default Special Price and Quantity discount but it gives an option to control special price and qty disocunt from a single place in a convenient way.
In a nutshell, X-Feepro is intended to create an Order Discount/Fee and X-Discount is intended to create a Product discount.