So what is the main difference between a knight and a mage? I read up on the descriptions of the two classes but I am not sure how to pick between them.

If it’s about picking between them and both descriptions call to you enough, you may want to see about their powers- Knowing vs Serving, or even basing yourself on their outfits if you like one more than the other! Really for something that personal it’s kind of up to you in the end.