When I got my 2nd Faye (first one being +Atk/-Res), I asked around for a suggestion of whether or not I should merge her into my newer one (+Def/-Atk). A lot of people seemed to have supported the idea of going with the former, so I kind of just took a leap of faith.
My first Faye was pretty okay at using double DD (and +3 Res/Fury, for whatever it's worth)...but that's the thing. She didn't stand out all that much, and especially given her ideal role. So I do the thing, and now I see she can be quite the tanking monster.
You're looking at a 56/55 Def/Res Faye. She was basically made for this.
People generally went for the former because her attack goes to 27 with an attack bane.
With neutral defense, your 56 defense would go down to 53 defense, and the res would be like 49, so while your defense and resistance wouldn't be AS high, they would still be up there, and her attack wouldn't have to suffer.
Basically, they both work, but an attack bane is still not recommended. If it works, then it works.
I think it's not to be understimated to heavily empaphise on a character's strengths. I'd rather have an unkillable Faye with badass burst damage than a sort "All-Rounder" attempt that can only survive one encounter. And it's not just the Def. -Res definitely takes a hit in her magic soaking, and especially when she's getting doubled.
True, it's about preferences, that's why I said the attack bane is not recommended. Out of the two you provided, she could become more powerful with the attack boon and become more of a specialized physical tank while being able to take some magic hits, but not become a "mixed tank" as well as she could with an attack bane.
Yes, she can tank better with the attack bane then she can with the resistance bane, but a lot of her damage would have to come from special triggers, pretty much like Felicia, and not everyone likes that style of play. That's why you take matters into your own hands. You asked a question and a group of people gave you input on what they recommended, which was to keep the resistance bane, but if you don't agree, then you don't have to.
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