leatherheadiowa How long are you staying in Athens? Sorry to say the Marriott would likely be my last choice due to location. It might be good for elite benefits but that would not be my main criteria especially if you are in town for a night or two. If it’s a special trip I would splurge for the GB or KG. The rooftop elite breakfast as the GB is very nice but there is no lounge.
The links rrgg provided are a good start to read up on the options.