If you run a heavy application for hours (think video editing) and the machine starts heating up, that's when the 28W vs. 15W will come in handy.
If you are only a light to moderate user, the distinction is not much. If you are a professional photo/video editor, then you might as well get the 15" model.
Get the nTB 2016, it's much better than the 13" 2015 and you won't regret it, unless you desperately need the dongles.
I have a MBP 13" nTB, Asus laptop (15W haswell CPU) and a MacBook Air 2011 - never ran into problems with performance.