About riven's build

Hey, so i play alot riven and it seems i still havent figure out the balance between CDR and AD, basically AD improves all her abilities but CDR let u use the abilities more often, for exmaple, i played againt riven in top, she went brutalizer - ionian and i went tiamat > brutalizer, even though i had more damage she could stun me more often and i believe she could kill me more easily in this game. so the question is, againt which matchups should i buy cdr(buy ioninan for ex) and which i should buy ad(rush tiamat or even hydra) ty in advance!