Should I level Quetz or Ozy?

Big sister: top-class for damaging, due to her multi buffs she can deal more damage than Ozy, particularly pouring critical damage in a specific turn of brave chain NP-x-B-e.
Dio: also pretty powerful for dealing damage to enemy, but could be slight lower than Quetz due to his attacking buff is narrowed to only "attack-up" (however, this could be improved by appropriate CE and team support)
But Ozy is quite good to combine his semi-support role using his 3rd skill. For instance, in a team of Ozy-Berserker-Waver/Merlin, the Berserker can overcome his/her poor np gain via Ozy and Waver/Merlin's skills, and Waver/Merin are also beneficial to release their NP faster.

My opinion is, if you already has good Caster supports, 2030, blonde anniversary mystic code, then Quetz is answer to kill enemy. But overall Ozy is slightly better choice due to his more versatile role than Quetz.

One more thing to add to the above—Ozy, skilled up, takes hits like a ruler half the time AND has a reliable heal.

So he’s much more durable than Quetz. She had a bigger burst but Ozy will outdamage her in the long run. He also makes RNG-based servants like Cleo and Hans much more consistent.

Both are great, but I think Ozy is more valuable in the long run.

Ozy, but let me explay why in my opinion he is the better servant ans so worthy of being leveled first
2 main reason.
1- he is more usefull in different situation, and in farm.
Ozy have a rare 20& group np charge up skill. very usefull for farming.
2:- Ozy is a tiny better then Quetz.
why
Quetz skill and stats are actually, a tiny better then ozy.
They both have the same carisma, ozy have the np for the team (and the very conditional buff chance up), but she have a buster up and guts.
You guys can say thet imperial privilage is better then her third skill , but on the same way imperial privilage lose a bit becouse it need ozy third skill.
what make her weaker is her skill uptime.
Except for her carisma, her second and third skill have long cd. Ozy buff are up 50% of the time.
thets all.