Comparing the X1 carbon or other ultra-portables in the X series with an XPS15 isn't really fair - and the 16GB is a limitation of Intels mobile ('U' series) processors, hence the same issue with Apple.
A fairer comparison to the 15" machine would be either the T580 or P51 series which has a similar choice of up to 32GB RAM and up to even Xeon processors if that's your bag - perhaps not quite as sleek as the Dell, but then it is a Thinkpad.