Slides vs Layers

I second what Adam said: "Use whatever suits you and your course."

It's partly a matter of preference. Programming layers takes more work. I think that's worth it for interactions. But, personally, I wouldn't want the bother of having to create custom Prev and Next triggers to move through dozens of layers that could just be separate slides. 

And, of course, it's still important to consider the user's experience. For example:

• I once encountered a multi-layered slide in which the layers completely covered the base. The content progressed in a way that felt like viewing new slides, yet the slide # stayed the same. And they hadn't programmed returning through the layers to the base... 
• I've also seen a tab interaction in which additional interactions were required within one of the tab layers. It got to the point where I wasn't sure where I was in the interaction anymore. 

BTW, I caution against using the built-in Prev and Next buttons to navigate through layers. Especially if the slides are numbered and/or the built-in Menu is used. Standard UI is that the Prev and Next buttons move to a slide with a different number and a different title in the Menu. 

Good luck with your Storyline journey!