Well, my guess is S1(w) > S2(w), since with the simple string "aaaa" there are two occurrences of the substring "aaa". The first is from position 0 to position 2. The second is from position 1 to position 3. There is overlap.
Note it is impossible to have two occurrences of substring "abc" using any 4 characters. Therefore, more substrings of "aaa" can also be found in the 10 character string.