Answer:
There is a slight difference but the result is the same.
In the notation ddx(y) the ddx part is an operator which says ‘the function which is the differential with respect to x of ⋯’ and y is the function being operated on. The ddx and y are two separate entities in this notation.
In the notation dydx we mean the function that is the result of differentiating the function y with respect to x. In this case, there is only a single entity: dydx.
Both notations mean ‘the function which is the differential of function y by x