I have a Dell XPS 13 and a Dell Precision 5520 [1], both 2017 models. It feels like the XPS gets around 8 hours of battery life and the Precision gets around 4.5 (even with the discrete GPU turned off). I got the high-res screen option on both, I hear that going with the low-res option boosts battery life.
I bought the Precision with very good specs, which may help explain why it gets worse performance than seems to be advertised. Its screen is also brighter than the XPS 13's.
The reason I got the Precision is that the XPS 13 was too small and slow and I didn't feel productive on it. I find that small screens really hamper my productivity, and having a smaller palm rest beneath the keyboard also makes typing more difficult.
System 76 laptops have very poor ergonomics and battery life, but great internals at a decent price. I bought a Gazelle Pro from them 6 years ago and it was basically unusable as a mobile computer; it got 1 hour of battery life out of the box and that declined to about 5 minutes after a year or so of wear (I'm not joking, it was bad). The keyboard was also terrible. I ended up using it as a portable desktop. That said, it was as powerful, if not more, than the XPS 13 that I bought 6 years later, and it cost slightly less too.
ThinkPads are also really nice. They have great keyboards and good battery life (generally coming with swappable batteries). I bought one about 8 years ago and it was probably the best personal laptop I've owned. The reason I went Dell this time around is because Dell ships machines with Linux pre-installed.
Macs are also nice. Macs have great build quality, great battery life, great keyboards (modulo touch bar), great screens, and decent internals. If Apple offered a high-end Macbook Pro without a touch bar, I would have bought that instead of the Precision.
If I have a chance, I might try running a battery life test on my XPS 13 and Precision 5520 and post the results here.
[1] The Precision 5520 is essentially just a rebranded XPS 15.