These are both oem and both of high quality. The difference in process has more to do with the amount of material required to handle the load.
So the 19s can handle the same load and at the same time keep the same weight at a larger size. the 18s in, my opinion, will perform the same if not better. The 19s are for looks. The 18s will also be more resistant to bending and cracking do to their smaller size. And the 18s cost significantly less!