Plate Helmets and Simple Helmets are set apart by the necessary research needing to be complete that can be used to make them both, on mostly unmodified game saves or basic scenarios you will already have the necessary requirements to make Simple Helmets, but on modified game saves or scenarios where you might have less research complete you will not be able to make Simple Helmets until you complete the research.

I think Plate Helmets also protect some more areas than the Simple ones? I could be wrong and I can't check right now, there may be some niche use cases where the plate helmet is better, like protecting the eyes when you have an armor gland