Squashing cookie balls vs slicing them
From a food science point of view, in terms of the quality of flavor and the texture of the cookies, there is little reason to choose slicing over squashing; both will give very similar outcomes.

You may find the aesthetics or appearance of the cookies will be slightly different with the two methods.   The sliced cookies will be more regular, especially if you cut them with dental floss; they will be quite round and even edged.  The squashed cookies will be more irregular at the edges and perhaps show more texture on top.