Ren'py by a mile. I can tolerate Unity for a great game I guess, but the company behind it is crazy and I'd encourage developers to not tie themselves to them. If you need more than a VN engine, biting the bullet and learning how to work with Godot seems preferable. At least you know they won't change the terms and extract all your money and rights to your firstborn all of a sudden.
The only Unity I can recall playing is Man of the House and it was such an annoying fucking grind that I hated it. Don't know if that engine lends itself more to sandboxes or not, but that one experience would have me hesitant. I'm used to RenPy by this point and can deal with its quirks.
Ren'Py all the way. It's really easy to mod it yourself and check out all the things you've missed and how to access them. It's like the game script itself doubles as a walkthrough.