According my little grammar book:
- 's is used for:
people or animals (the girl's name, the horse's mouth)
organizations (the government's decision)
time expressions (next week's exam, yesterday's milk)
after a
singular noun (
my sister's room)
- of" for:
things (the roof of the house, the title of the book)
long word constructions (What's the title of the awesome new movie?)
Hope that helps.
