Saba is sweet; the balsamic adds the sour. There's no specific recipe on line or on the video, so I just eyeball everything and adjust it along the way. Sometimes I find myself adding more saba and balsamic along the way.
Grape juice will probably do fine, probably even if you add sugar. IIRC, saba is grape must barrel aged for years on end. It's a thick syrup. Mosto cotto or vin cotto are other names for it, or something that is a close cousin.
Real saba is complex, fruity and it really does add to the flavor of the sauce.
This is a robust sauce so there's room to play with flavors, but use saba if you can.