The style you mention would traditionally have coved corners. This won’t look right. Plus, your ceilings look low. You could do a plain moulding. I will look for some inspirational photos. Bbl

There seems to be two schools of thought. A traditional large molding stained dark color or very small cove molding painted same color as walls and ceiling. Since it appears you have new sheetrock and not sure about texturing, I think you would benefit from looking for traditional methods especially if resale is important to you. Talk to some agents in the area to see what buyers want. Be guided by that. I don’t see many original features so probably can take whatever path you like.

Opps. Here is more traditional look. You squared off archways so that feature is gone.

What is the ceiling height? What will the rest of the trim package be? (Doors, baseboards, etc)

I don't think crown moulding is usually included in most Mexican home or a lot of Spanish and Spanish revival homes in the U.S. So, if you want to stay true to the style I wouldn't include crown mouldings - at least now crown mouldings used in most American homes.

I agree with Norwood this is not a feature in a spanish home as the walls were adobe and the only ceiling to wall transition was just the adobe curved. I think we need to know more about your style that is going to be in this house and as mentioned you have already squared off the arches .