The penthouse is an absolutely great place to stay.
It is big enough to stay with a group. You have a great view over Athens.
The penthouse is located 4 mins. walk towards the metro, and with the metro you are within 10 minutes in the centrum. Or you take a taxi, it doesn't cost much and they drive you in 15 minutes to the centrum.
Mister Dimitris is a verry nice host. Everything is aranged perfectly.
In short, if you want to have a good time in Athens and want to stay in a great place, you should definitely book the 360° penthouse!