I got a Lenovo 15.6" 3840x2160 2-in-1. 1080P is perfectly doable, but I can see a difference in sharpness with 15 inches. Windows 10 is good about scaling. I previously had a 4K monitor at my old work (which was using Windows 7). Windows 7 wasn't so good with scaling apps that didn't support hiDPI (it's a feature that's present in current issues of Adobe software...and CS6 for Mac). The only app I've encountered on my new system that doesn't scale as well is Color Munki. I don't use it often, so I haven't tried a hack which involves making a manifest file for it. I'd say the only main weakness of UHD is that it can consume more battery power. For photo editing, I think it's nice. With higher resolutions, it's easier to see DOF (and can be more revealing about whether something is in focus).
As for choosing the Dell Precision, I'd say it's worth it if you do video editing or 3D animation work. For Photoshop/Lightroom it can be overkill. I got a Dell precision mobile workstation a few years ago for 3D work (and to compliment a desktop workstation I built from parts ordered at Parts Express). The precision is still going strong, and is a beast at rendering (thanks to a Quadro card). I no longer try to take it with me on trips or meetings, since the battery life is now almost non-existent. I now have my 2-in-1 for that. If you're not doing 3D or video work, I'd say the Dell XPS is a great jack of all trades laptop.