You can use either… FCC used to recommend c9.io but then they required a credit card to be entered for the free plan, so Glitch is recommended now.
Glitch is pretty cool, and its really quick to get set up.
Plus with Glitch, you get a public url so your project can be both developed and hosted their as well (I think) so no need to push to Heroku to host your project once your finished. If you develop on c9, you will then need to host your project somewhere else to submit it.