The difference between these two is that the first one describes a tree in an extremely consistent form whereby parens represent children and spaces represent siblings. The latter represent a tree only by means of a special "parser", which converts this sequence of characters into the tree.

What's special about the former one is that it composes perfectly with any other tree structure using the same textural representation of trees, where the latter one is only useful in isolation - one cannot use this representation inside some other arbitrary character sequences which is to be converted to trees, because the means by which it is parsed into a tree does not compose well (unambiguously) with other means of parsing sequences of characters into trees.

Perhaps the cleverest thing about S-expressions is that they were invented before we even had a theory of parsing unambiguously followed by a proof that we cannot compose multiple of these unambiguous parsers in a way which produces another unambigous parser.