Ask HN: Why do you use Bootstrap or Foundation?

I've been using Bootstrap since version 1 and have enjoyed the process by which I build sites with it. But recently I've been wondering why so many of my friends have moved to Foundation. Most of them preach that it's "faster" and "slimmer" but I want to get a better idea from those of you out there that have been using either for a while. What's your take on it?

When I build web-apps I build them with as little overhead as possible so I start off with the BS grid, nav, buttons and forms. If I need anything else, I'll add it later. Why would that be any different than Foundation?