It's a fine laptop, and the XPS is clearly "better", but as you say, given your usage parameters, the Yoga sounds like a good machine for you. I'm the same way w the XPS 13 vs 15. I could really use the 15's power, but portability is one of the top two requirements on my list, as I'm damn near always on the go. So the 13 it is.
The Yoga 900 gets pretty good reviews. There were a bunch of problems with last years model, but Lenovo, to its credit, has addressed most of them. The only knock on it is that the trackpad is not great. But if you are doing something with extensive cursor use, you can just get a cheap bluetooth mouse and problem solved. No reason not to get it.
I wouldn't get the orange. You'll get sick of it. While Black, grey, or silver are admittedly boring, they age well. Color not as much. Personal experience. The colors, especially around the corners, tend to wear off over time, and the machine consequently looks shabby. Even more so if you do a lot of travel.