[Aganzir]

I like both too. Nobody can deny Nerdanel's achievements, but Thuringwethil is either

a female messenger, probably some kind of a Maia - and cool enough to use imagination in choosing her form

or

the name Lúthien took for herself after leaving Tol-in-Gaurhoth, and no one needs to be told of Lúthien's extraordinary achievements. Still, if it's her, I probably like her Thuringwethil plot the best out of everything she did.