Just ordered a Macbook Pro 2015 with dGPU, and the Dell didn't tempt me, but made me cautious. I may admit that the recent hike in price, and buying the computer from a third party retailer at a good price, made the decision easy. Still I dream of something along the line of the experience I have on my Macbook. I don't hesitate to bring it with me, and it does everything except...being efficient on heavy tasks.
The problem with usb c is it's infancy and lack of options to connect devices to it. The computers electric capacity is too low to power anything else than a single device. The adapters have been bashed hard on review. Look at the strings of problem this port has on the current Macbook line.
http://www.apple.com/shop/reviews/MJ1K2AM/A/usb-c-digital-av-multiport-adapter
Another reason for buying the Pro is the multitude of ports, and the sea of possibilities to connect todays devices to your computer without buying experimental and expensive adapters. Who cares about what may work tomorrow, if you can have your cake and eat it right now.
A SD reader is a must. I miss that more than I would like to admit on the Macbook 12. Since this computer was released over half a year ago, the aftermarket hasn't launched any of their accessories to the masses. As stated earlier, the young nature of USB C is making it irrelevant without an USB C to "normal" USB adapter. Again I really hate the bulkiness of the current Pro, but at the moment it is the only desktop replacement on the market with the ports and the power to make me want to ditch both iMacs or Mac Pro.
The PC laptop market who deliver dGPUs are also very much aware that their market is mostly Windows gamers, so the need to pursue that type og customer is eminent for a brand like Dell. Tried Windows for a while now, but I have to admit, all I do in Windows is to play FIFA, GTA and some random FPS. Very few of those titles are released for Mac or OS X. Most games are not that CPU dependent, but rely on great graphics power. Again, very different from applications for productivity that utilise the CPU. Movie editing is the only Mac work I would consider dependent on GPU.