IMHO is Ella, no doubts.
Alix was thinner, or better more slender, i'd say; moreover, i've already seen this gown on Ella, but never on Alix; finally, her face is definitively Ella's, not Alix's; i'd post some photo to comparison, but i don't have time now (my parents in law are waiting for me for lunch

)
I'd say the have their own interests to say it's Alix: a photo of Alix's would attract more customers, 'cause she was in a certain way more famous; and those who aren't expert, and can't recognize them - they're not passionate as we are! - maybe just want to own an important photo... i hope i could explain myself ^^
But, in conclusion IMHO is Ella.