By List, you actually tell, that your object implements List interface only, but you don't specify what class your object belongs to.
By ArrayList, you specify that your object class is a resizable-array.
So, the first version makes your code more flexible in future.
Class ArrayList - Resizable-array implementation of the List interface.
Interface List - An ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted.
Array - container object that holds a fixed number of values of a single type.