The answer to question "What is better, Bootstrap or Foundation" is that it depends. Here are a few pointers to make this decision a little easier to make.
Grid System
Foundation started having the upper hand by allowing responsiveness with percentages first, but since Bootstrap added the .row-fluid class this difference has disappeared.
Sizing Units
In order to calculate typography Bootstrap uses pixels while Foundation uses rems. If you are more comfortable working with pixels then Bootstrap is your better option.
Features
Foundation:
- Built in validation provided by Abide
- Relies on media queries to load responsive content for different browsers
- Right-to-Left support
- Off-canvas navigation
- Tours
- Pricing tables
Bootstrap:
- Productivity oriented, its default behaviors are more fine tuned to get you started faster.
- Responsive Embeds makes easy to incorporate responsiveness to different elements.
Ability to Customize
Bootstrap has a more recognizable default look that can make a little harder to make a site look different from the rest. On the other hand, Foundation has a default appearance closer to the browser specific look, which means that custom styling can be done easier even though it takes a little longer.
Mobile
Foundation was created with mobile-first in mind so it's more suited for this kind of design pattern. Bootstrap is still a little more desktop-oriented. But as commercial project are almost always designed desktop-first, that should
Browser Support and Performance
There are not really noticeable performance differences between these two frameworks. The biggest difference is that Foundation does not support IE8. If IE legacy support is important for your project then Bootstrap is your best option.
Community and Support
Here, as expected, Bootstrap has the upper hand. It has a larger developer community and a wealth of plugins and widgets that can be used to extend its functionality.