We have been strong proponents of Bootstrap since it first came into the world. Ever since it was called Twitter's Bootstrap.
The most beautiful platform that outdoes itself with every product release. We started building all our websites on Bootstrap 4 even when it was just on Beta 1. We may even have a website running on Alpha. That's how much we believe in Bootstrap.
In our past individual experimentations and need to try other solutions we had dived into Foundation. Those were the days were we were doing extensive reading on Foundation 4 vs. Bootstrap 3. Foundation has the upper hand on certain components of Bootstrap but I guess its just a personal preference. What you feel most comfortable with and what feels just right. For us it's been Bootstrap!
Please note we have a plan to update all our client websites to Boostrap 4 production release when the final production version is released. Just because a site at the time was built on an Alpha or Beta doesn't mean it remains on that release perpetually.