Sorry, meant to add that although some people will tell you that oregano and marjoram are the same, and they are indeed close relatives, they are not the same.
However, oregano (Origanum vulgare) is often referred to as marjoram (Origanum majorana), despite being a much more pungent herb. In regions where oregano and marjoram are both known as marjoram, the term 'sweet' is used to distinguish marjoram, which is indeed much sweeter than oregano. I think that just about makes sense?!?!
I think the difference would be when using oregano as a preventative treatment that the pungency potentially increases the content of essential oils.