Hello from Wisconsin,
My wife and I both found Istanbul such a stunning city we wouldn't trade a day away for a one shot day in Ephesus. But we are a couple. And we have little fear of walking around in cities on our own. If you are more comfortable in a tour group, then maybe two tours of Istanbul and one of Ephesus. Or three of Istanbul.
The Aya Sofia is such an amazing building only getting into it once did not satisfy us. First off, it is beautiful. Second it is enormous. The largest roofed/domed building for 9 centuries. Did I mention that it is beautiful and that from the second floor people are almost like ants. From the second floor!. Imagine that.
Plus the water cisterns. The Arkeologic Museum. Think of this. The city of Istanbul is an arkeologic city. Imagine what they have in the museum that THEY think is old and noticeable. You should see what it is sitting outside of the museum exposed to the weather because it isn't good enough to get in.
You might consider a tour on one side of the Golden Horn and one on the other side. Check a map. the Golden Horn separates two sections of the European side of Istanbul. And a third on the other side of the Bosphorus. Step into Asia. It is a ferry ride away.
Ephesus is a pretty amazing ruin of a library. But it is a one pony circus. It is stunning that Ephesus was a sea port and yet today you can't sea the sea form the hillside. That is worth something. But Istanbul...my goodness. Even the Little Aya Sofia is worth seeing.