For me personally, I just let them both have the name. It suits both of them for very distinct reasons. Lilith Vatore because she's a vampire and that name represents the vampire Queen who supposedly is the Mother of all vampires, the very first one. Lilith Pleasant because her name is iconic to TS2 and represents a contrast to her twin. So for you, I guess it would come down to which one do you see yourself playing with more? Do you play with vampires more or will you play with the Lilith in Britechester more?
Do you see them both frequently? Or do you see one or the other more? Have they ever been in the same world together when any of your Sims were around? I would figure out which one of the two bothers you the most by having the name because another Sim has it and change the name for that Sim.
Even though I didn't vote in your poll because I'm keeping both as Lilith, if I had to change one, it would be Lilith Pleasant. My reasoning is because she's a TS2 Sim and while I do have fond memories of her in that iteration, Lilith Vatore was made for TS4 and is now iconic to this iteration. Also, while I like TS4 version of the twins, they're not the 'real' Angela and Lilith in my mind. Just like a lot of Simmers don't perceive The Goths as the 'real' Goths. Does that make sense? But as said, for me personally, I'm keeping them both Lilith. :)