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.
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.
webpop looks awesome by the way.