I realize the main sights in Athens are in a relatively small area. However, my wife has some mobility issues, and it might best for us to use a hop on hop off bus to get the "lay of the land" after we arrive. Does anyone have experience with either City Sightseeing Athens or Athens Open Tour? I'd like to hear about your experience and would be particularly interested in the frequency of buses.
All Hop On -Hop Off bus services in Athens are similar.
I have never used these buses but being often in Athens, when I see them sometimes I find it a relaxing way to get around town, other times I feel sorry for the passengers stuck in the middle of traffic, noise, heat and pollution of the main avenues.
And as the bus companies specify in their documentation: "it gives the passengers the opportunity to save energy for the museums, art galleries, heritage and archaeological sites".
Which means that anyway, if you want to visit the sites and museums you will have to walk.
As you realized, the Acropolis, the Roman Agora, Hadrian's Library, the Acropolis Museum, the Parliament, and other main places to visit that I forget are all in the same pedestrian area accessible from only one of these bus stops.
No means of transport other than your feet will take you to visit them.
If it were my first time in Athens I would suggest a guided city walking tour as you will get a lot more out of that. You will most likely be sightseeing mostly in the areas right around the acropolis and the hop buses probably aren't going right there as they are small and pedestrian roads mostly. You will find much of the areas to see will be areas you will need to walk to. Hop on /off buses are good in cities where things are spread out like in London, Paris, Rome etc.. But you will find that most of what you want to see will be clustered.
Myself I've been to Athens before and greatly prefer to use the sub-way to get to an area and then explore and then walk. The sub-ways are inexpensive, fast and clean with lots of stations around the parameter of the Acropolis, Agoura Philopappos Hill, Plaka and other areas you will likely want to see. These areas tend to be more pedestrian oriented.