Time-wise it's about the same by train and by car/bus. There are 6 trains daily costing ~45€ (one way) or ~25€ (one way, slower night train) and 9 buses (~45 regular price). By private car one has to add ~31 € (one way) to the cost of gas/petrol for the 504 km between Athens and Thessaloniki. The train company says that the travel time will be reduced from 5.5 hours to less than 4 later this year. At that time I think that, other than the frequency of flights (>15 daily) there will be hardly any advantage for flyers...