Square didn't annouce on which engine that theyre using, which guys do you prefer?
Do you think that Luminous would cause a longer development of the game or faster release with Unreal Engine 4?
If we're talking about what is probable, SE are likely using Luminous since Remi Driancourt of Advanced Technology Division (people in charge of game engine creation and testing) hinted that, besides XV, there exist other games in development using Luminous, and that the company would share more information in the future.
As to which engine I'd prefer, I echo @LeonBlade's comment.
I mostly prefer Luminuous Engine, with all the resources and time invested in it, SE better put some use to it aside from FFXV, if not it would be a waste of efforts.