We’ve been to both Thessaloniki and Patras twice in the last decade. We enjoyed Patras but it doesn’t really compare with Thessaloniki. Although it’s the third largest city in Greece it’s about half the size of Thessaloniki by population and has nothing like the range of museums and sites.
Some good points: the centre is pedestrianised with a pleasant bustle of city life, the Roman theatre is a beautiful (but small) site, and the area around Trion Navarchon has a neighbourhood feel with a range of pleasant local restaurants and bars.
On the downside it seems too have turned its back on the sea. The front of the city is mainly blocked from the sea by an old railway line and you have to walk to the city’s edge before you get the possibility of a seaside stroll.
I’m not necessarily trying to put you off. We went back because we enjoyed it, and may well go again. Essentially though it’s a small provincial town rather than a city and you can walk the three or four main streets in not much over an hour.
Taken as a small town it’s a pleasant place to wander with the usual advantages of anywhere in Greece - friendly people, a relaxed atmosphere and nice food.
Alan