I can't specifically attest to the Expedia family of sites, but every OTA I've booked through thus far has received the 5X as expected.

If it codes as a travel agency, you will get 5X. If you don't yet have Prestige, but do have another Citi Mastercard of some sort, you can easily check how the site codes by making some kind of refundable booking and checking how the charge coded immediately on the list of pending charges. Hopefully that can then help you decide if you would benefit from getting the card.