I was always concerned about the seal on the plastic kids and infection potential with them. I've had the plastic lids leak. You can boil the metal ones to sterilize and if they can leak, microorganisms can enter the mason jar.
True..... I just like that the plastic ones clean up so much better. Plus, you usually can't crank down on the lid anyway as pressure will build up in the jars....so, it generally is not an air tight seal anyway.