What's the Difference?: Middle Grade vs Young Adult

Here in my neck of the woods, the grass is under a blanket of snow, the materials budget is in hand, and summer reading is right around the corner. Early spring is a great time to prune and grow the library collection and prepare for the influx of summer readers. With that in mind, the question what's the dividing line between Middle Grade and Young Adult fiction comes up from time to time. How do you determine where it fits? Do you look at the age of the protaganist? The subject matter? Or do you use other benchmarks like which collection it will circulate best in and reach more potential readers?

As in so many things, professional judgment and knowledge of our community of users plays a big part. From a writer's perspective, here's an article from the August 2014 issue of Writer's Digest that talks about the key differences between MG and YA. Do you agree?