This assumes the primary reason you are using a framework is to get the niceness of an infinitely nesting responsive grid and you care about semantic HTML.
Putting the grid in your LESS or SCSS is so much nicer than littering your HTML with "large-4 columns", etc -
I'm currently using Bootstrap in two very different projects. One is an internal admin site where we have managed to get very far just using Bootstrap components in the HTML without any custom CSS at all. It's an AngularJS project and with Angular directives for Bootstrap (http://angular-ui.github.io/bootstrap/) you can use Bootstrap like any native UI toolkit and if you don't need a customized look and feel you can get by with the defaults or just add a Bootstrap theme (http://bootswatch.com/) on top.
Another one is a large application with lots of LESS stylesheets and themed for different clients. By building our custom UI components in the same way as Bootstrap components are built we have managed to keep it very easy to customize for different themes. Bootstrap provides whole lot of variables as customization points and we have added a bunch of our own on top. By changing those variables we can create new themes very easily.
For choosing Bootstrap vs Foundation, just pick one you like. More importantly, use the components provided by the framework as much as you can. When you to roll your own components, make them in the same modular fashion as the framework components. Put components in their own files and namespace them with classes. This way you can keep things nice and modular, and your stylesheets don't turn into an entangled monolithic blob that is difficult to maintain.
As far as foundation vs bootstrap I've used both tend to prefer foundation.