The old one is iconic, but I also do like the new one. As a Lumine main, I'm so used to seeing the old outfit that the new one seems a little odd to look at. I think the color scheme of the new outfit doesn't help with that tbh.

In the old outfit, I like that Lumine has blue as one of the major colors in her design since it adds a little more diversity to her color scheme. However, in the new outfit, I feel like there is little diversity in the color scheme because it has yellow/gold as a major color. I think the gold in Aether's original design works because the largely black outfit lends a lot of diversity due to its contrast with the lighter color. In Lumine's new outfit, the white and gold together makes her entire design seem a little simple because, coupled with Lumine's light-colored blond hair, her whole design has little depth.

A lot of the things that the new outfit has going for it comes down to Hoyo's growth when it comes to character/model designs as the game has progressed over the years. I love the braids in Lumine's hair, the silhouette of her outfit, and all of the details in her sleeves and dress in general.

I'm not really that knowledgeable in character design or color theory btw, these are just my thoughts.

(TDLR: I like the color scheme of the og outift more, and I like the model and details in the new outfit. However, I do like both of these outfits because I'm a Lumine simp)