There are many UI frameworks out there - Bootstrap, Foundation, Semantic UI etc. I have used Bootstrap in the past, however I'm very much inclined towards using Material Design for Angular. There are many good reasons to use Material Design over Bootstrap or any other framework that you might consider.
When I say Material Design, I'm referring to Material Design for Angular - https://material.angularjs.org/. I have been using Angular in my applications and I've made that as a defacto framework to use for any front-end work I get. I don't consider any library to use in my project unless it also has an Angular implementation. Material Design for Angular and UI bootstrap, both are such libraries. So why choose Material Design over Bootstrap? I did search for this and couldn't see the comparison in terms of pros and cons or why one framework is more suited over the other. Thus, I'm jotting down my thoughts on why Material Design wins over Bootstrap.
Material Design delivers the wow effect; the click of a button, or selecting a checkbox, entering form details and everything else in the specification delivers great user experience. Bootstrap on the other hand is good if you are building very simple applications that look like they are made only to deliver the functionality and great user experience is not of high priority.
CSS3 introduced FlexBox, which allows you to create layouts with ease. Even if I'm not inclined towards great user experience, I would chose Material Design for this feature only. Bootstrap uses floats for its grid system. I hate floats; floats were designed to wrap text around images. Floats come with a clear fix and adding more mark up or CSS for clear fix is like carrying extra luggage. Using display: inline-block is another alternative, but it expects you to write the markup in a certain way or add margin-left: -4px.
FlexBox should be used instead of floats or inline-blocks. I can't imagine writing my applications using anything other than FlexBox.
Material design provides several directives for managing layout, creating drop downs, form fields etc. The syntax used here is more semantic. Consider this code snippet:
The above code snippet declares that you are creating a row and it has three elements with attribute 'flex' added to it. This will create a three column equal width layout. Please note, there's no extra CSS or class added to this. The markup is clean and who doesn't like clean markup.
In case of Bootstrap one will have to add classes to build the layout, I wouldn't like to copy the code and paste it, you can see it for yourself here - http://getbootstrap.com/css/#grid
Whilst both Material Design and Bootstrap provide you the ways to create responsive layouts, I prefer using Material Design. You only have to add certain attributes to the markup to change how it looks on different devices. For instance the above row based layout can be converted to a column based layout for a mobile device by adding layout-sm attribute:
Bootstrap uses classes instead.
Many would rather complain saying that Bootstrap is widely used and Material Design is not. That's true, because Material Design is new and you can't expect every Bootstrap website to be converted to Material Design in no time. The other reason that I've heard on not using Material Design is that one will have to learn it before becoming productive and it involves a big learning curve. I'm of the opinion that this is not true, it didn't even take me a day to get started with Material Design. Also, if you take a look at the syntax it's almost the same except that you would use directives instead of classes in Material Design.
When choosing a technology or a framework, there's no right or wrong. There are opinions and perspectives. As a developer you should have the freedom to chose what you like. However, I would advise that don't be afraid to try out something new. Don't constraint yourself to one library; it's always fun and a good learning experience when trying out different libraries.