Quetzalcoatl and Merlin

Not going to tell you who to pick, but I'll provide some information that maybe you didn't know here.

Merlin is limited and as far as I can tell only appears in 2 summon campaigns (https://grandorder.wiki/Merlin/Campaigns).

Quetz is not limited (but is story locked) and it looks like she only has one where there is a good chance of getting her (https://grandorder.wiki/Quetzalcoatl/Campaigns), but since she's non-limited there's always the chance of a spook.

I'm only NA but from what I've read they are both good servants gameplay wise but Merlin is game breakingly good.

Merlin make your Buster team more versatile and Stronk.
2030 Merlin make your team OP.
Quet + Merlin is also an OP comp for riber.
Off course Quet is a excellent rider and I lover her character. But normally a support is more critical. Since for DPS, you have more options, like Ozy, or even Lv90 Kintoki. Quet's 2nd skill can be a support for berskeres, but it is better for herself to make a combo of (1)-2-3 skills and NP-B damaging.

And Quet is theologically can be summoned in every poll, but not Merlin as limited.

There will be another summoning campaign with Merlin as a single rate-up sometime next year. As far as I know, Quetz will never get another single rate-up again, she'll always be sharing with other riders after the Babylonia banner I think. But she's not limited so she can spook you. Merlin is better for gameplay reasons. I'm not gonna recommend one over the other though, you can decide for yourself.