The short answer: Zeline
The long answer:
Hero selection and grading is inaccurate when it is taken out of context. Which is better Zeline or Lianna? It depends on the situation, but each has their individual strengths.
Your context makes more sense for Lianna. You have sonya and caedmon, and so the buff clearing ability of Zeline is not as pressing. Zeline is fast and the attack debuff is quite nice for survivability, but Lianna’s ability to remove one enemy is also key if you were charging one opponent and she needs to disappear or be neutralized.
It sounds like you are not consistently fighting 4100 team power opponents, given that your heroes are either working toward 3^70 or are 4 stars. The game is a bit different there, and Zeline’s buff clearing skill is more common on the 4 stars. I’m thinking you would likely benefit from the flexibility of having Lianna and Zeline at 3^70 and may even like Lianna more than Zeline.
Your question however was “Zeline or Lianna?” and I’m going to interpret that as “who should I use my precious potions on when i finally collect 6 of them this summer?”
Your place in the game will likely be different there. Magni and Marjana may be full grown and you might be fighting more strong teams where you can’t bring Sonya and Caedmon. Nobody can argue with Lianna, but you might find Zeline is better when you are fighting buff heavy heroes like Ares or Vivianna. Also, I believe Zeline will be more effective at defense because the attack debuff being a fast skill is super annoying and keeps going off.
In that case, you may find Zeline to be more valuable at 4^80 than Lianna.
But in truth, you might like Lianna more than Zeline as well- best defense is strong offense right? This is too close to get a clear obvious decision, but if I had to pick one, I’d pick Zeline.