In almost all cases, it’s more appropriate to use Browserify and npm over Bower. It is simply a better packaging solution for front-end apps than Bower is. At Spotify, we use npm to package entire web modules (html, css, js) and it works very well.
Bower brands itself as the package manager for the web. It would be awesome if this was true – a package manager that made my life better as a front-end developer would be awesome. The problem is that Bower offers no specialized tooling for the purpose. It offers NO tooling that I know of that npm doesn’t, and especially none that is specifically useful for front-end developers. There is simply no benefit for a front-end developer to use Bower over npm.
We should stop using bower and consolidate around npm. Thankfully, that is what is happening:
With browserify or webpack, it becomes super-easy to concatenate all your modules into big minified files, which is awesome for performance, especially for mobile devices. Not so with Bower, which will require significantly more labor to get the same effect.
npm also offers you the ability to use multiple versions of modules simultaneously. If you have not done much application development, this might initially strike you as a bad thing, but once you’ve gone through a few bouts of Dependency hell you will realize that having the ability to have multiple versions of one module is a pretty darn great feature. Note that npm includes a very handy dedupe tool that automatically makes sure that you only use two versions of a module if you actually have to – if two modules both can use the same version of one module, they will. But if they can’t, you have a very handy out.
npm is most commonly used for managing Node.js modules, but it works for the front-end too when combined with Browserify and/or
$ npm dedupe.
Bower is created solely for the front-end and is optimized with that in mind. The biggest difference is that npm does nested dependency tree (size heavy) while Bower requires a flat dependency tree (puts the burden of dependency resolution on the user).
A nested dependency tree means that your dependencies can have their own dependencies which can have their own, and so on. This is really great on the server where you don’t have to care much about space and latency. It lets you not have to care about dependency conflicts as all your dependencies use e.g. their own version of Underscore. This obviously doesn’t work that well on the front-end. Imagine a site having to download three copies of jQuery.
The reason many projects use both is that they use Bower for front-end packages and npm for developer tools like Yeoman, Grunt, Gulp, JSHint, CoffeeScript, etc.
All package managers have many downsides. You just have to pick which you can live with.