I hear Whitney is pretty rare, but I would personally go with Bunnie! She's so sweet and cute and pretty rare herself. I haven't seen her as much as thought I would in ACNL...
;w; Whitney is such a cool wolf. I had her in my old town for a while, and though I cycled her out, I wish I kept her for longer. She's beautiful.
I have Bunnie in my town. She's a sweetheart, and I like peppies a lot. She's adorable, and I have Dotty, too.
I don't know how to choose since snooties and peppies are bae. Bunnie should be easier to get than Whitney, though, so maybe set your sights on Bunnie, especially if you're letting go of Flora, another peppy.
I think it depends on your current villagers, like if you already have a snooty but not a peppy after Flora leaves maybe Whitney wouldn't be a wise choice and such. But with the current information I'd say Whitney, but that's just because I like her.