I'm not a huge fan of Hillary- the Clintons come to mind!!

Ari could work for a boy or girl- although if you had a girl you could always just put Ariane or Arielle on a birth certificate and call her Ari from birth anyway. No-one needs to know except you guys- that way she has the choice to use the longer version on a job application/formal papers in the future should she wish. Nothing wrong with Ari- although I feel it does have a nickname ring to it. I prefer the 'Arie' spelling for a girl.

Since you don't plan on having a 3rd- definitely use Ari no matter what. You obviously love it- it is a unique name and I fear you will regret it if you don't use it!

Best Wishes!