The XPS just feels a bit smaller, but perhaps that's because the bezel on the display is tiny in comparison.
Build quality on both machines is generally excellent (notwithstanding MBP keyboard problems that have troubled some). I don't think the display on the XPS is that far behind in use (I have the FHD display, rather than the fancier one). Having done lots of tweaking in X, I think mine looks great with Arch running i3.
As others have mentioned, the smaller trackpad on the XPS isn't quite as good.
Goes without saying I suppose, but a big consideration might be whether you have buy-in on or would have interest in various OSX companion apps that are useful as a developer (Alfred, Kapeli Dash, etc). Also, if you have any intention to develop iOS or dual-platform apps then the Mac's XCode and iOS simulator would require you to go in that direction.
One thing I would mention is that if you're going ultralight with the XPS to develop on the move, then the higher capacity Power Companion is really useful piece of gear to keep you going [1]
Much lighter and slimmer than the Hyperjuice brick I used to hike around with for the Mac.
[1] https://www.amazon.com/Dell-Power-Companion-18000-WCKF2/dp/B...
They touchbar, if you opt for one, is pretty much useless other than to unlock the macbook. Fiddling with the volume and the bright switch is a pain.
The track pad is superior in the macbook.
The screen resolution, is superior on the macbook, unless you go for a 4k display xps at the cost of battery life.