The W520 was and is an outlier in just about every category, though. Not exactly a fair comparison...
Also, if you're going to be talking about the form factor and weight, you should probably mention the power adapter...The XPS takes a 45W, 0.2 pound little USB C device. The W520 requires you to lug around a 1.7 lbs, 170W beast that definitely qualifies as a brick. That and a lack of support for the latest NVMe drives are some of the only shortcomings, IMO.
The power adapter is a good point! It really is a beast.
Including the AC adapter, the XPS 15 is 40% lighter than the W520 for roughly equal performance and similar battery life. Not too bad.
It's still a bit sad though. After 5.5 years I moved to a macbook pro 13" 2016 and the performance was a drastic downgrade from it (the w520 finally broke...).
6.5 years without a DRAM increase for 15" laptops is bonkers.
It was $1500 for the laptop, it wasn't even crazy expensive back then.
On top of that it even supported 32GB. All in a rather sleek 15" form factor. (6.2 pounds vs 2018 XPS 15 4.6 pounds).
Sources: https://forums.lenovo.com/t5/ThinkPad-P-and-W-Series-Mobile/...
https://www.youtube.com/watch?v=Ih3rD2ccGkM