Morellonomicon or Nashor's tooth on fizz and ekko

I wanna ask you guys what you prefer between this two items on these champions.I go more for on them because i find this item more synergize with them as has W which both passive an active deal AP damage via basic attacks and have really good scaling (45% AP passive) and (35% Active) while has passive like with 80% ap scaling and his W passive like fizz deals both ap and missing health damage.Negative side of item is you'll run out of mana faster but with two you should not have mana problems.Nashor's also helps you take down carry if you're on cooldown.