Which one you will use and why: Twitter Bootstrap or Foundation?

As per my recommendation i prefer Zurb Foundation Front End Development Framework.
due to major reasons i followed.
More semantic classes: this is of course a matter of opinion, but for us this can shave time off of a build.
A golden standard: You can hand F4 to just about anyone with html and css knowledge and you will see VERY similar markup for the same site from nearly all.
This means it becomes much easier to contract out help if need be, and ensure quality design.
The UI: Yes Bootstrap wins this if you judge on it alone, but if you look at it as a whole really the UI of foundation is very close.
Plus lets keep in mind that you can customize any of it with app.css Zepto: I am a fan.
What can I say.
SEO is part of our world, and fast loads mean something, and even leaving SEO out....fast loads = less bounce.
SEO: Google has already come out and recommended responsive design for mobile SEO.
Why not?: What site can I not recreate with the framework.
It is still CSS.
It does force us to rethink how we code, but isn't that why we love to code in the first place? Figuring out solutions to simplify design? Foundation Framework have these major features.
CSS Preprocessor : SASS Grids: Fluid Mobile Visibility Classess: Feature-based IE Browser Support: IE8 Available Wordpress Themes: More than50 Commercial/Free

Bootstrap for the simple reason that I find it easier when using React.

Well, the answer to your question may be more subjective. It depends on what you want out of a particular framework.Foundation is an stable framework. However, Bootstrap has a beautiful, clean and neat UI/UX. It also goes very well with angular.js.Off course, you can always attach your normalization style sheet with it (I did). You can also edit CSS in SASS with Bootstrap. it does not limit you.

I think both are good framework but i mostly use Bootsrap as due to regualr use in projects i feel more comfortable with Bootstrap but its dont mean Foundation its bad at all ,