I've used both extensively. Foundation uses SASS, is more modular in approach, is much easier to re-skin, and has a better grid system. Bootstrap uses LESS, h

I've used both extensively. Foundation uses SASS, is more modular in approach, is much easier to re-skin, and has a better grid system. Bootstrap uses LESS, has much more mind share, more plugin and theme support, and has better default styling. Both are great frameworks. I tend to use Bootstrap for prototyping, and Foundation for production.