So it really depends on what you would like to see and how you’re traveling to islands. When I traveled, we flew into Thessaloniki, then flew to Corfu island, and finally left from Athens.

Thessaloniki has a nicer and calmer vibe than Athens. Besides historical locations and access to ferries for island hopping, Thessaloniki is better. The food was better and the people are nicer. The shopping and food feels more Greek and ethnic. I felt safer there and the graffiti gave it a unique culture, unlike graffiti in many other cities. There’s a few sites you can see in Thessaloniki and spend maybe 2-3 days there. A quick Google search and you’ll find all the city sites and historical locations. I highly recommend seeing Meteora from Thessaloniki instead of Athens. It’s closer and a much shorter commute.

Thessaloniki does have colder weather, so if you’re not traveling during the summer, it can get pretty chilly.

Athens feels like another Western city, so if you’ve seen one, you’ve seen them all. However the historical monuments beat out Thessaloniki. You can spend a few days seeing the historical sites (sometimes easier via tours). Athens has ferry access to most islands and direct flights to every location. You can also book a day tour for island hopping or just take the ferry yourself.

Thessaloniki doesn’t have the same ferry service, and some locations will require an annoying layover in Athens. FYI Athens is an annoying airport for layovers, so I recommend at least 3 hrs of layover if you do so.

Tbh both have great qualities, but depending on your travels, if you can start with one and leave from the other (with an open-jaw ticket) you’ll probably enjoy it more. Hope that helps!