I wear my mismatched (dark and light) Tahitian earrings much more often than my white Hanadama studs. Ombre' pearls go with everything, even other white pearls. I vote for the ombre'.
From your writing you are just wowed by the ombre'. You're not saying the same thing about the white necklace. Just that it is comparible in price and quality. You will wear something you love much more often than something you think won't go out of style.
For what it's worth, I am drooling over for that ombre. (Oh, I just see that I'm late to the discussion. In that case, I'm still drooling for that ombre, AND am so happy for you for your choice. I know you're going to love it.)