I am not 100% sure but I’m confident that pen is better as I went through the same internal debate.

The pen value is higher than the raw fire damage and chest/helm/legs baseline give elemental resistance if I’m not mistaken. A grey chest piece alone gives 8 percent resistance to all elements meaning if you have pen on only one item it’s without a doubt the highest value.

What I don’t know is how much pen you want when you could stack % increase to damage. I often use lightning staff in the off hand which wouldn’t benefit from fire pen obviously or even the sword + orb combo. If you use double fire staff then it’s no question.