Any modern dual-core should be good enough for Xcode, it's just longer build times. Since both 13" and 15" are limited to 16Gb RAM, it won't help with running larger/multiple VMs either. Heck, I've done a lot of work on the fly with Surface Pro 4 in VS.NET, and it was fine.
Screen real estate is the big one for me, and the price difference between 13" and 15" is really not that great when looking at higher specs though...