Apart from the display, the specs seems bit weaker on the Dell - the Macbook Pro has more storage capacity, and a faster processor even in the base configuration.
Also the Macbook Pro 15" now has the ForceTouch track pad, which will be more useful over time (and Apple makes excellent trackpads anyway, Force Touch or not).
I have a Macbook Pro 15" Retina currently, that I use in non-scaled mode (so I get 1:1 use of the pixels). I'm not really sure how much better the higher resolution would look on that small a display.