Who is a better initiator?

Both are pretty terrible initiators.

Your goal with either is to wait until an opponents are distracted, alone, or separated and trying to pick them off without taking any damage. After the clean kill, you slink back into the shadows and spread paranoia.

"Initiation" is a capacity used more to initiate a team-fight and lock down a priority target. Most initiators also lean pretty heavy into the CC/utility direction and lack a lot of damage, relying more on boosting champions like Rengar or Kha'Zix for an epic dunk rather than trying to finish a target off themselves. The job of an initiator is to start a fight, not end one. Building a lot of bulk also adds a lot of power to an initiation and initiator, creating a catch-22 where any cooldowns and damage directed at the initiator is not directed at there nearby allies. If an initiator is squishy enough to eradicate the moment they start a fight they don't get to create widespread panic and disrupt enemy formations by jumping on a hostile team.

Under no circumstance should a Rengar or Kha'Zix really aim to initiate a fight. Both are optimally played with damage-heavy builds and the ultimate goal of eradicating an enemy before they can react.