Bootstrap:

☛ Bootstrap offers unlimited number of UI elements

☛ Bootstraps uses pixels

☛ Bootstrap encourages to design for both desktop and mobile.

☛ Bootstrap support LESS as its preprocessor

Foundation:

☛ In Foundation UI element options are very limited in numbers

☛ Foundation use REMs

☛ Foundation encourages to design mobile first

☛ Foundation support Sass and Compass as its preprocessor