Top of the line XPS 15, including touch screen is $1650:
- i5 Quad Core (i5-6300HQ)
- 8GB RAM (upgradeable to 32)
- 256GB HD (upgradeable)
- 2GB Graphics card (GTX 960M)
Lowest quality new model MBP (15" w/ touch bar) is $2399:
- i7 quad core
- 16GB RAM (not upgradeable)
- 256GB HD (not upgradeable)
- 2GB Graphics card (Radeon Pro 450)
So for $750 you get a little more RAM, slightly better CPU, and Mac build quality (I have no idea how the graphics cards compare). That seems like a pretty big difference to me for fairly similar specs. Especially considering that you can upgrade your XPS in the future.
I'm on a Macbook Air now, but for my next laptop I'll be deciding between the 2 above (or possibly the last model rMBP).