The leather is died with a vegetable tannin. Furthermore, from EK:
Once assembled the surface of
the leather is then treated with moisturizing/waterproofing product from
Italy called Noacqua and then finally the edge of the palm rest (at the
seam) is treated with a acrylic-based sealant for durability.
Real leather will get dirty quicker than fake leather, this is why it is often applied in VERY high quality headsets, I mean fake leather. Real leather is an option if you want it, but fake leather does better overall because its easier to care for.
So as always there is no best thing. Touching and sweating into some plastic kind of thing an ultimately really badly tanned leather isn't the best thing one can do, and its probably poisonous to some degree.