Considering that Howard's Hyborian Age was a fantasy mythological world that he envisaged around the time of Plato's Atlantis, I've chosen to go for ancient armies rather than a high medieval look.
My version of Aquilonia is a mix of greek and roman styles and my Cimmerians have a sort of celtic barbarian look.
It's all fairly subjective and can be interpreted in many different ways - as others have said, even Howard wasn't consistent with the look and times of his created empires.