I've used them both on big enterprise software products and they are six and one-half dozen to me. There is not a wrong choice, they both kickass in a lot of ways and have their own small frustrations and annoyances.
I haven't used Foundation as much, but both frameworks (BS4 and Foundation 6) are designed around Sass components that can be customized and recompiled. BS3 has a very good Sass port. Overall, both frameworks are bit opinionated, but very customizable. If you want complete flexibility, you can look at other Sass frameworks, such as Bourbon.
Foundation had the lead, but now I'm at the point where Bootstrap is 99% as good and more simple to set up, and if I really want the complexity that I used to use Foundation for I'll just use Bourbon + Neat.