Programming: 13" or 15" TB?

Dual core is fine for programming. But, of course more power is better. Screen real estate is a bigger issue. XCode does a lot with visual editors and these take up a screen space. I find a 13" to be too cramped for me and I have to hide too many windows. The 15" is better.

But most of the time I use an external monitor or two.

 

13" is fine but the extra screen space and the extra 2 cores makes things smoother on the 15"

 

Dual core is fine. If your project gets large you will appreciate more. If you're running an external monitor then the 13" will not be an issue. I run a large Swift project and even a MacBook display works well (e.g. when traveling - it is noticeably slower, though quite usable). 16GB is ample for me. I've got Xcode, Sketch (with a 445MB file open), Outlook, Safari, Slack, and the Simulator all running and Activity Monitor tells me I'm using a hair under 13GB.