Well, personally I think MS is using a wrong strategy for Surface Book. I think they are still testing the waters just like Tesla Motor Company is using phases for the roadster/model S for inventory--flagging a high price with low stock first then so on for the stable consumer models.
In terms for specific reasons "bang for the buck" you've asked, the Surface Book from reports haven't be reliable/stable and for pure perspective of hardware, MS dropped the ball somehow for the price (which fairly to say is common in 1st gen products). Such as:
1) Slow writing SSDs, by opting for the Samsung PM951 the performance of the device is on par with the storage size. Most people bought the i5 256gb model, and it manages a 500mb/s for an NVMe. In comparison, other laptops on NVMe managed 700mbs if not higher than 1000mbs.
2) Could've be better on GPU or power, for the price a 940m (1 Gb) isn't very justifiable and it actually bottlenecks performance if you're a video/graphics editor. The industry standard for base model laptops is at least a nvidia 960m with 2gb of memory anyways for premium devices. Another thing to point out, when using the default power brick for heavy task, it actually won't charge and even drops. Which is bad for default, and the only solution is to buy the bigger Surface Dock to manage it.
3) Other than that, there are a few minor annoyances like tacky/wobbling hinge and a better I/O port is needed (or at least drop the price for the Dock price/add-in usb type C)
For OP though, I have to say the Dell XPS 15 is better than the 13 for personal reasons such more I/O ports and real estate. One thing Dell or MS should've implemented is a battery charge limiter, since they're both sealed-in batteries it's beneficial for battery life (just like Samsung, Lenovo, etc). I'm glad you've decided, anyways. The Dell XPS line with the infinity display is light and fast, one of the best currently.