With the univisa, you can go in and out of Botswana but only for a day trip- since you are staying overnight, you would need a new visa on your return.
If you have no intention of visiting the Zambian side of the falls, then a double entry Zim visa is probably the least expensive option. However, if you also intend to visit the Zambian side, then you could get a univisa first, and then a single entry Zim visa on your return (or vice versa, depending on when you intend to do your Zambia visit).
Thank you! That was my understanding of it as well.