My rMBP is only 256GB, and while it's "enough" - only barely. Having to think about disk space regularly and make sure to remove software I'm not using regularly is something that I've not had to do in a very long time. It's tight enough that I'm hesitant to dual-boot Windows with Boot Camp like I have on every other Mac I've owned for gaming and such.
I've been hearing it for 20 years from Mac users: "Why would you want XYZ feature anyway?"
It's hilarious and there is no wonder in my mind as to why the vast, vast majority of businesses use Windows and not Mac.
Well, I can't speak to most businesses but every corporate I've worked at in the last few years has been characterised by an SOE that involves strangely spec'd machines eg. only 4GB RAM but 1TB HDD which is 95% empty but thrashes nicely when swapping (just running browser with a few tabs, office and outlook). It will also most likely be an i7 that never gets to top out, meanwhile the underwhelming wifi infrastructure (not PC specific) means boot times (due to profile and policy replication) are in the order of minutes. Oh, and regardless of how old they are they never get upgraded (eg extra RAM, switch to SSD) because the bureaucracy disincentivises doing so.
At least they have a configuration to choose from! If they shopped Apple, they'd get 3 choices: Cheap overpriced crap that they have no control over, overpriced crap that they have no control over and Expensive overpriced crap that they have no control over. But hey, it would be shiny! They'd then just have to build their own infrastructure to manage their fleet of Macs like Google did because nobody else builds stuff like that for Macs since nobody uses them in business.
True, this is a real issue, and I agree we're paying premium_price^2 for the newest MBPs.
However, in this day and age of networked computing and fast WiFi why would 256 GB not be enough? Did you check the performance of the XPS SSD compared to the MBP?
I take it the RAM is DDR4. The 16 GB LPDDR3 RAM means more battery life. The resolution of the 2015 MBP 15" is also in between 1080p and 4k, a sacrifice, for battery power.
The graphics card isn't upgradeable on either machine. And I did look at the XPS. I'm not going to buy a machine which makes a lot of noise, and the XPS has coil whine. Why don't you compare to the 2015 rMBP? I tried out the butterfly keyboard of the 2016 version in store and I found it far too noisy (the travel also isn't a good aspect). Do you really need the graphics card? Have you looked at eGPU?