Bootstrap's styles are more polished and comprehensive than Foundation's. Foundation's CSS file is very well commented and half the size of Bootstrap's. Bootstrap's modals work with IE8, with Foundation's modal I get an error. Foundation's Orbit JS slideshow is easy to implement, but seems a little buggy in FF.

phpnode got it right - I'd be more inclined to use Bootstrap in cases for which I have no design, or a simple design with latitude for many elements. Client extranet, back-end utilities, etc.