First of all, since i don't speak English, pardon me if i say something that doesn't make sense.
"snotty,bratty,kind of vain", if you see her that way then conan, Heiji and other detectives are also snotty, bratty and kind of vain. It's just Gosho's way of portraiting smart characters, i myself get used to it.
The reason that i favor Ai above all others is because she's different, different from any anime girls i ever know. She isn't the typical "cheerful, way too kind, childish" like Ran, in fact, because Ran has too many good things about her without a noticeable flaw make me grow tired of her. It would be nice to meet such flawless person like her in real life, but this is manga, it very boring to read about such character, to a level that i could almost predict how she gonna react in certain situations, it's just no fun. Ran is really typical, if you read other mangas, you could find other characters who has similar personality to her. She only shine when Shinichi around, without him, she's just a normal high school girl with no special background.
Ai is much more interesting, she's mature, cool and kind, and i love her sarcasm, it make the series more funny. Outside, she look like a cold person but if you pay more attention to her, you'll find her as sweet as Ran. Unlike Ran, she is independent from Shinichi and has her own life, her own problems, and her own world with people involved in it. She did rely on him when confronting BO, but it's because they're too strong for her to fight alone. The reason she so terrified of them not because she scared of death( she attempt to suicide before) but because she afraid the BO will kill everyone she know, including Ran, if they ever found out about her identity, plus she's changing, she's stronger now (Even Conan admit it).
Well, i still has a lot of things to talk about Ran and Ai, but i'm beat now
So maybe some other times ...