Heh, I've actually used both of those adapters on a few clients' XPS 13 laptops. They're pretty good. I'll be curious about the compatibility of the Dell adapter with the MacBook. Please report back on that.
(Nobody makes a single dongle that has everything, so I'm instead getting two different dongles that each seem to do useful things.)
The Dell WD15 dock does have everything I wanted. The problem is my monitor. It only has two connections on it, dual-link DVI and VGA, and the DVI is the one that gets me the full resolution. But I guess there's no such thing as mini displayport to dual link DVI, unless I'm not thinking about this setup correctly, in which case someone please correct me. It's frustrating because I used to have this problem with lesser video cards on older computers, but now I supposedly have a connector capable of 4K resolutions, but it can't do this in between one.
Please, I'm hoping someone will say "no, dummy, this $10-30 cable/adapter will get you what you want!" So...there's your queue?