Which guided tour should I choose for Athens?

For the tour you are looking for I recommend booking them directly with Alternative Athens rather than with Viator which is a third party operator (and it will be cheaper)

The principle of all the tours you mention consists of a group of 15 to 20 people with headsets walking behind a guide and listening to his comments and instructions.

So, as for the choice, it's up to you to know what you want to do and visit and how much time and Euros you want to spend.

The difference is Acropolis + Agora + one or two more sites vs. Acropolis + museum. I think the first one is more appealing especially given that I just learned that Acropolis museum ticket is separate from Acropolis combo ticket.

Viator is an American company. Alternative Athens is a Greek company. Do you think it is less secure than Viator?

Viator is an American company. Alternative Athens is a Greek company. Do you think it is less secure than Viator?

NO. Since it's a local company, you will get better service from them should something go wrong that from Viator.
I think you need a change in attitude. US does not mean better or more secure. European companies know how to secure their websites just as well as US based companies. In fact, EU countries have tighter and stricter laws on privacy and require more consent from you to use or share your personal data you give to their websites than the US.

“Do you think it is less secure than Viator?”

It's not a question of secure or not secure, Viator does not organize the tours that you mention, it is an intermediary who will buy the tickets mainly from alternativeathens.com and resell them to you for a few Euros more.

I don't understand what you mean with this "Acropolis combo". You don't need the Combo ticket with the guided tours that you found on Viator. You need only the “Acropolis & Slope” and “Ancient Agora” tickets.

The Combo ticket includes entry to 5 other sites which are not part of the tours. (this would be impossible in a single day anyway)

The Acropolis Museum being a separate ticket office not dependent on the Ministry of Culture

hey hey jyung
you are so head strong on using viator, which is a consolidator of tours. you have got many answers to your many questions about tours. go with a local company.
withlocals.com/athens
culinarybackstreets.com/athens
airbnb.com/experiences/athens
matt barretts
athensguide.com or greektravel.com
scroll down to the different tours that they offer. open up your blinders with many tours/companies out there. you can also stop at the tourist kiosks in city centers to see what they have available. with the cost of you trip it's worth looking at other tours and what's not the cheapest, you get what you pay for. you can do a walking tour, a food/market tour which is so much fun. have done many in the different cities we have been in and some you are planning to visit.
many on this site do their own tours without using viator. hope you find something that suits your fancy and enjoy. good luck
aloha

I'm not sure a guided tour is needed for any of those sites. I walked around the Acropolis and the Ancient Agora listening to the Rick Steves audio tours on my headphones. The Acropolis Museum is absolutely fascinating and not to be missed. The Roman ruins underneath and the top floor were my favorite parts. If the weather is nice I recommend the cafe for lunch.