I love Phoebe, I'd stick with her! She moved into my town last summer, and I totally adore her! I love the uchi personality, and her house is super gorgeous inside and out.
As someone who has had both in their town for a long time, I prefer Phoebe. Her design is just more flashy and interesting to me. Deirdre is cute looking, but there isn't much more to her than that in my opinion.
I've had both. I personally like Deirdre better. IMO, the uchi personality fits her better and feels more authentic I guess? I get where people are saying that Phoebe's design is cooler, because yeah it is. But I just like Deirdre better for whatever reason ^^ If you have Phoebe now and she's not cutting it for you, then I suggest meeting Deirdre in game and deciding from there~
It?s hard too decide between those two, but I`d go with Phoebe because of her special design, based on a mythical phoenix. Deirdre is a nice villager for a natural town though, maybe you could decide on considering your town theme.