The copper rings in a PEMF mat are able to provide a pure magnetic field, which is why most PEMF manufacturers think that copper rings are preferable to wire mesh.
Most mats have either 6 or 8 copper rings, and a mat with 8 copper rings can cover a greater area than a mat with 6 copper rings.
Also, if you do much traveling, a full body mat that folds into fourths is easier to put into a suitcase than one that folds into thirds.