Author here. Over the years, Zurb's grid has consistently stayed ahead of Bootstrap's in functionality (ex: Zurb was allowing embeddable grids while Bootstrap was still using fixed width) and also in class clarity: "three large columns" vs "col-lg-3".

Beyond these reasons, though, i wouldn't suggest other developers turn their back on Bootstrap if that is what they are familiar with. Its adoption has reached a point where the resources surrounding it are unmatched.

If you are comfortable with bootstrap, stick with it. If you are like me and have a bit of contrarian in you, give Foundation a try and see if you prefer it.