The "7 Best" article has a great list, but I think they missed the mark on both the description of Sketch and some of the ratings they gave it in the comparison chart.
Their description states:
"It is similar to Photoshop in many ways, allowing you to edit and manipulate photos.Many of the tools are similar for both, but it is easier to use than Photoshop."
This makes it seem like software aimed at photo manipulation. It's not. I'm not sure I'd say it is comparable to Photoshop really in any way, especially not in purpose.
I also learned how to do enough to make wireframes in Sketch in about two hours when I was also brand new to OSX, so my personal experience is that the learning curve is pretty shallow.
It's more like Milton Bradley's Othello, in my opinion. "A minute to learn. A lifetime to master."
I found it easier to use out of the gate than both Axure and Balsamiq, though neither of those were particularly hard, either.
By the way @scrow9, I know you're planning on taking a class in UX (or are currently). Most of these options offer student discounts. I'd ask your instructor before purchasing anything.