The main differences - all layout now responsive so no need to include bootstrap-responsive.css and use row-fluid, container-fluid. All stuff already always fluid now.

Second main point - the framework now mobile ready - that mean a lot of mobile optimisations and widgets for mobile devices.

Foundation was made to support as many screen sizes as possible with minimal fuzz and they're using Sass with Compass. They released version 4 about 5 weeks ago and it is mobile-first. See http://foundation.zurb.com/ .

OK.