Neither are even remotely used where I am, but I prefer
Anaïs. It just has a better ring to it, and the pronunciation just rolls of the tongue.
Thaïs seems a bit incomplete and short to me for some reason.
Zelimkhan | Anzor | Ruslan | Anwar | Rizwan
Zainab | Sitora | Madina | Ruya | Sezim
My PNL: https://www.behindthename.com/pnl/159719/117411