I lol'd at the title. Just having a Gruntfile is the first smell I notice when looking at a project. Seriously, why take two simple tools and layer a unnecessary "task runner" on top of them?
I chose Grunt because Make generally requires a lot more background knowledge and ongoing discipline. Most Grunt tasks are configured by relatively constrained JSON data, so it's pretty hard for non-experts to muck it up unless they need to write a custom task. A Makefile would have created much greater education burden for me. That's particularly a concern when the team's Make/shell expert has time-limited involvement with the project.