Greek temperatures vary more by altitude and exposure than by north to south. There are only slight climatic differences from the islands to the far north. Height above sea level and exposure to ocean breezes make more difference. Both have good weather and fewer crowds. Most things are open full blast then.
Winter is winter.
Greece is not a tropical country, its too far north. In winter it is dark, cold, rainy and windy. That can have a stark beauty, but you aren't going to sit on the beach all day in that weather. Cross River Press, New York, 1992. Also available via Cosmos Publishing of the Hellenic Literature Society .