Haylie/Hilary Duff - Neither. Haylie is a butterface in my opinion and I need to see a better view of Hilary post-baby bellybutton.
Kate/Rooney Mara - Kate. Don't find Rooney the least bit attractive (but then again, I only know her from Girl with the Dragon Tattoo, and she was made to look ugly in that.)
Emily/Zooey Deschanel - Zooey, even if she is a Katy Perry clone, and you can't recognized her without her hair on her forehead.
Dakota/Elle Fanning - Elle has been much more giving to us.
Jessica/Ashlee Simpson - Couldn't care less about either of them, but Jessica has been good to us, so I'll go with her.
Rosanna/Patricia Arquette - I've barely seen Rosanna in anything other than Pulp Fiction, so Patrica (who was better in True Romance anyway).
Vanessa/Stella Hudgens - Vanessa is hotter, but like ppfd said, Stella is without a ring. Tie.
Kaili/Dani/Bella Thorne - Bella is the most mainstream, and the most navel friendly, so her.
Kate/Pippa Middleton - Kate is someone's wife and mother, so I'll go with the single Pippa.
Kendall/Kylie Jenner - As far as I'm concerned, one is a hottie with a beautifully unique bellybutton, and the other is a troll with a generic innie. No contest. Kendall.
**I suppose we could do mother/daughter combos as well:**
Susan Sarandon/Eva Amurri Martino - Eva has been good to us during her short career.
Goldie Hawn/Kate Hudson - Kate, based solely on her post baby outie.
Melanie Griffith/Dakota Johnson - Neither.
Demi Moore/Rumer Willis - Demi, because Rumer inherited her father's chin.
Lea Thompson/Zoey Deutch - I grew up in the 80s, so Lea.
Heather Locklear/Ava Sambora - Couldn't care less about Ava, so Heather.
I guess in ten years we can do Katie Holmes vs. Suri Cruise and/or Angelina Jolie vs. Shiloh Jolie-Pitt.
**Or THREE generations:**
Priscilla Presley/Lisa Marie Presley/Riley Keough - Priscilla, based on her bellybutton alone.
**Or even aunts/nieces:**
Julia/Emma Roberts - Emma for her body, but I'd rather see Julia acting.
Jane/Bridget Fonda - Jane, back when she was young and hot, although Bridget was good in Jackie Brown.