Hi Plientje! Welcome and may you have a fun, safe trip to my home state.
I'm guessing that individual stores will vary from location to location, but overall, the Michaels in my town is far superior to Joann's. I wish it were the other way around, since the J store is much closer to me than M's. I *do* go to both, since they carry the same types of things and often I can use a coupon. I think J's deals/sales tend to be more frequent and better than M's. It seems that whatever I buy at J's turns out to be less at the register than it was marked on the shelves.
My J's only carries Fimo and SculpeyIII...no Premo. So, when I need clay, I go to M's to get Premo. M's has professional art supplies (paint, etc.) while my J's just has crafty kinds of things. I would say that M's has a far better selection of tools and extras (e.g., metal leaf) than does J. In fairness, my J store is dedicated over half to fabrics, etc. so there's just less room for crafts.
So, have fun, and tell us about your experiences!