JSPM is not as mature as webpack, and its ecosystem is not as rich. But it has some cool features, such as being able to load a module from a remote url, which is not possible with webpack, even webpack 2 — at least yet.
Edit: I published react-css-variables, which makes defining CSS variables easy using a React component.
CSS variables are a game changer, they allow easy and cleanly separated JS-CSS communication (among other things !).
A great article popped up recently, To Designers With Love (A Letter From a Front-end Developer), explaining why it is important that designers should design in a way that helps front end developers getting sense of their design.