I was just in Santorini on Dec. 29 - 31 and I can say that it is truly stunning. I would check the big cruise schedules if possible and avoid that but in the spring you should still be fine. I haven't been to Capri, so I can't advocate Santorini over that, but we really did love it and wish we'd had more time there. It does look just like the postcards. We stayed in Oia at Alexander's and it was very nice, right on the cliffs, great views and a lovely breakfast served at our leisure every morning by the kind and efficient Assia. It was a great experience.