It's a never ending story if you seek for latest and greatest (there's always a newer & better Mac coming), ask yourself, what you really need. If your work required better processing and graphics and your current setup is showing the age maybe by then, it's time to move on. Mind you 13" consider a downgrade considering your current 15" has Radeon 6XXX series in it.

So the crucial questions is, what kinda task you do on the Mac?

For the money of getting a new Mac, I would get a kick-ass thunderbolt storage and a decent external screen to work with.
Rumours says there will be a retina Cinema Display coming. Or you can settle with some Dell UHD one. They are all good. You will cry to work on a 13" (even it's retina) workspace, when all your palettes and menu already filled up 50-70% on the screen.