When launching our isomorphic app earlier in 2015, we were surprised by the size of the bundle Browserify created. After reluctantly trying Webpack, the build was inexplicably 1/3rd the size.
The best I can surmise is that Webpack did a better job identifying duplicate dependencies (React, React Addons, etc. were always problematic it seems) and de-duped successfully.
Now, my reason for using Webpack is primarily for the insanely simple code-splitting.