Linda, in June I spent three nights on Santorini, then three nights on Naxos, and enjoyed them both. They are very different, including especially that Santorini has a very active cruise port (not sure how active in October), while Naxos has no cruise port, just ferries. The ferry in between the two islands is just 2 to 3 hours, so certainly you could split the time if you chose to. I also enjoy small towns and villages, and visited some on both islands. On Santorini, one day I took at very good bus tour which included the Akrotiri ruins site, some beach stops, a winery with tasting and views, and a final stop at Oia for sunset. One stop was the beautiful very small town of Megalochori, which I enjoyed so much that I returned the next day by public bus. I spent a couple hours wandering the town, had a delicious lunch, one of my most memorable meals in Greece, and then went on to the nearby larger town of Pyrgos for another couple of hours. Great day. On Naxos, where the main town is a lot of fun with many great restaurants and shops, plus an interesting old town, old castle, port area and nearby beaches. I arranged for a guide with a car to spend a day taking me around the rural part of the island, including some very enjoyable small towns and villages, old churches and monasteries, marble quarries, crafts people, etc. Another great day. I don't know your plans for moving on to Salzburg, but obviously you would need to mesh your last day(s) into those travel plans, without cutting things to close. Personally, unless you want to stay and relax at a nice hotel with a real swimming pool, I think Santorini could get a little old after six or seven days.