Both are based on solid resets and (now) great, nestable, responsive grids.
Bootstrap is great for when you're not a designer and you need to get something out there that looks passable. It includes a lot of fully styled elements and have very nice typography and forms out of the box. I found Bootstrap a lifesaver to get something up at http://www.eventsourcehq.com quickly.
Foundation is great to use as the foundation of a website when you have design skills or are working with a designer. Using the grids and the defaults gives you a lot less to think about when getting the rough outline of a website done, but you won't have to start overriding or deleting a bunch of included styles once you want to apply your own. We've used Foundation as the base for our example project template for http://www.webpop.com and plan on making more foundation based templates.
In short:
Bootstrap when you want something styled out of the box. Foundation when you just want to quickly get the structure and layout going and then apply your own styles.