I like Sixt, I rent a lot from them, and am a Sixt platinum card holder. And I would probably use this branch again. But be aware that this isn't like a Sixt branch in Western Europe. Firstly, the Audi I rented was clean and in good condition - but old (64000 km) and didn't, for example, have a USB port to charge a phone. Secondly, the check in process is slow, despite me having booked via the app. They still require lots of paperwork which they have to photocopy and you need to sign. It also took them thirty minutes to find a working credit card terminal to take my deposit. Finally, you get a very hard sell about taking out full insurance. You're told how bad Greek drivers are, how parked cars are often scratched etc etc. In the end, I took out fully comp insurance for everything not because I thought I would do anything to the car (I didn't) but because I felt I didn't trust the branch to not make damage up.
So, my advice would be to take out fully comp insurance, just factor that into the price even if you don't need it, and pay everything in advance to reduce the time taken at the counter. And bring a spare charge pack for your phone!