I've used both Makefiles and Grunt for projects and I much prefer Grunt:

* I find the js syntax of gruntfiles much cleaner and harmonious with the js code I'm writing for my projects

* From what I've seen, Makefiles are meant to create output files - seems like Grunt can be set up more easily for things meant for display (linting, etc)

* Better ecosystem with grunt plugins: https://github.com/cowboy/grunt/blob/master/docs/plugins.md