I'd opt for the 4k Dell screen and tbh, my experiences with Windows vs. OS X, is that If find Windows easier on my eyes. A couple of for instances.
In Excel, I typically have to increase the zoom up to 125 to 150% in OS X because for what ever reason its being rendered too small. In Windows, its fine. Likewise in the mail app the text is too small. All things being equal, with resolution and what not I find Windows easier on my old eyes YMMV and as such my experiences may not be typical but I do find its easier.
At this point I find that the MBP wins over the dell in:
- Looks, and I will not discount how that can affect purchase decisions - the MBP does look really nice. Gorgeous lines.
- The Apple ecosystem, that is if you're already in the apple ecosystem, then the MBP isn't a bad decision.
- Customer Support, I've not had bad experiences with dell in the past, but I've had better experiences with Apple
What I like about the Dell is not just that it beats out the MBP on spec, but the keyboard is better, the components are replaceable. I don't have to spend over 200 dollars and get a new top case because I needed to replace the battery.