Both amazing, but I'd choose Cappadocia.

The question is how much time do you have? From Istanbul, you can do a long day and fly to visit Ephesus. You need at least two very full days to see much of Cappadocia - plus flight time. Ephesus is beautiful and very interesting, but if I had the time, I would probably choose Cappadocia because it is so unique. I would not do Cappadocia as a day trip.

What are your interests?

Here are some recent decisions about both locations:

http://www.fodors.com/community/europe/missing-ephesus.cfm

http://www.fodors.com/community/europe/turkey-istanbul-cappadocia.cfm

And then you can deduce an answer by applying a hyspothetical syllogism.

We have abt 3d2n. Or max 4d3d. Love scenic places, less of museums & history!

Exactly.

Had the same issue and it really depends on your interest. For our family, we've wanted to see Ephesus for awhile and are history buffs.

Unless you really love ruins, I would choose Cappadocia, because it's very different from everything you are going to see in the world.

I don't think you'd go wrong either way, but if you are more interested in scenery than in museums and history, and if you have 3 days (or even 4), Cappadocia would seem a reasonable choice. If that is what you choose, note that the Goreme and Zelve Outdoor Museums are incredibly scenic. And consider taking a hot air balloon ride!

Cappadoica is scenic.

My vote would be for Cappadocia. I had four days and three nights and it worked out great, although I wouldn't have minded having another day. Ephesus was also great but one day was plenty.

So you see...Ephesus is half a day I reckon , Cappadocia needs 3-4 days. If you go to Ephesus might as well go up to Pamukkale too..
Should help you decide.....