Overall, I think the SB2 15 is a better buy than the 13. The GTX 1060 is much less common in notebooks, except as noted by others above in gaming systems (which are often under $2000). But gaming systems typically are much bulkier, they usually have terrible battery life, their displays are usually average at best (unless you spend closer to $3000), and they're designed for flat-out performance and so they're usually very hot and loud.
To get a productivity notebook with a GTX 1060 is much less common. Asus was going to put a GTX 1060 in the new Zenbook Pro UX550, but for some reason they scaled it back to a GTX 1050 Ti. Really, the SB2 15 is in a special class because of the display quality, battery life, and overall form factor. There's not another 2-in-1 like it. So I can see spending $3000 on an SB2 15 where I would never spend $2000 (or whatever) on a SB2 13. If I really needed a GTX 1050 in a 2-in-1, I'd get the Lenovo Yoga 720 15.
To me, the only real downside to the SB2 15 is that it's not 4K. It's sufficiently high resolution, but it's not going to play 4K video, which a machine like the Spectre x360 15 will do, and very nicely. As a writer, though, I'd have to opt for the 3:2 aspect ratio and just live with less than 4K video (which is really only some Netflix content in Edge right now, anyways). I'll probably bite the bullet and buy the 16GB/512GB version for $2800 or whatever. It's a lot of money, but there's just not another system that I know about that will meet all of my needs the way the SB2 15 will do.