I prefer Selcuk, personally. It has more "town" overall. I found Sirince to be lovely but a bit too remote, and if there is one piece of advice I can give, it is this: get to Ephesus a half hour before it opens and be the first person through the gates because the tour groups are going to be brutal, and they start from the upper slope. If you get there early, you can see the entire place, empty, as we did, which is unspeakably beautiful.
You have received good advice, generally. I might add that there is no need for Kusadasi, it was a low point in my many weeks in Turkey, very touristic, although the water is pretty, the beach is very built up and narrow and packed in summer. Better? When in Istanbul, take a ferry up past Rumeli Castle and check out the Black Sea, which is beautiful. Not turquoise water, but really, sublime, plus there is good food up that way and plenty to see. Just a thought. Or find beaches outside of Kusadasi.
My hands-down favorite part of Turkey was Pamakkule, personally, although I didn't make it to Cappadoccia. It's four hours, driving, from Izmir, although there are tours. Spent two nights and loved it. Amazing ruins, plus a calcified mountain and lots and lots of local culture. All of Turkey is outstanding, I think. Istanbul is quite stunning as well, and I spent ten days there, in total.