I do not understand the Helm comparisons. Other than speed they really do not play a similar style at all.
Athanasiou obviously has vastly superior hands and more physical size. His skating is also more elusive and 'east-west' style.
Helm has very good defensive abilities, Athanasiou has always been very bad in that regard. I would also say Helm has a higher hockey IQ.
Neither player is a particularly good playmaker.
Athanasiou is very, very good at specific things, but mediocre to very bad at other things. This makes him an intriguing prospect, but will also stop him from becoming a star.
I would value him around a high 2nd to late 1st.