I voted for turn-based, because I think it's the most strategically rich options. As others have noted, XCOM and Fire Emblem are great examples of very popular strategy games using turn-based mechanics, and I've always been a big fan. As I mentioned elsewhere, I'm surprised that a AAA title in 2020 has the cajones to go with turn based combat (I guess the reaction in some quarters shows how ballsy a call that is), but I couldn't be more delighted.
That said, I'd hardly be up in arms if they went pausable real time a la Dragon Age, full ARPG, or any other system you could name. I've played great games using a ton of different systems, and you can enjoy each one for what it does well or uniquely compared to others. I think BG3 could have been made using any of the above, and would still have had a chance at being a great game depending on the strength of execution plus all the other factors like story, character, faithfullness to FR lore, etc.
I'm very excited for this turn based BG3!