Elven magic workshops give 38% fewer supplies per 3 hour then non-magic one in chapter 3. However, they do take less population and culture and you may not have upgraded your workshops to their highest level while in chapter 3. Chapter 4 magic ones give 58% more supplies then the non-magic ones, but the pop and culture cost is slightly more than non-magic ones.
Once you hit chapter 5 through chapter 14, 5 magic workshops produce as much as 9 non-magic ones. 5 of them will also take less pop. and culture than 9 non-magic ones. This not only gives the space back for 4 workshops, but can also reduce the number of residents needed to support them. Since progress through the early chapters is also fairly quick, you will need a constant supply of blueprints or pay the 300 diamonds to upgrade them. If you use diamonds and start with chapter 3 ones, the cost is actually 650 instead of 590 by waiting. So, I think it's better to wait. In fact, I tend to think for elves it's best to wait until chapter 5 for magic workshops and residences. Although workshops are better earlier for humans, residences aren't and I'd still wait for chapter 4 for workshops and chapter 5 for residences
I have a spreadsheet which compares both residents and workshops between non-magic and magic ones through chapter 14. Basically, 5 magic workshops are as good as 9 non-magic ones and 3 magic residences are as good as 5 non-magic ones. The workshops overall are a better deal since they also save on population requirements. I hope this is a good starting point for you. And here is the link:
Non-magic vs Magic Residences and Workshops.
Remember, though it's your game, so look at the info and make the best decision for your playstyle.
