For me it's simple. Price. I'm a hobbyist and I'm not (yet??) good enough to exploit any technical differences between 5DIII & 6D. I wanted to move to Full Frame from an entry level 1000D that'd I've had for 4 years and learnt a lot with. The GPS & Wifi work for me and are useful, I don't care about a second card slot.
As Helen Sotiriadis (who's a far better photographer than I'll ever be) says in her review of her new 6D:
this camera has only one card slot but, in all honesty, i don’t miss the second slot. i’ve never had a card fail on me (and i don’t treat them very well) and i’m conscientious about backing up my files as soon as possible, so the probability of losing a shoot is minimal. i also speak from the perspective of not having to shoot once-in-a-lifetime events. i don’t do weddings or christenings to be too nervous about it... still, in previous years, nobody had double slots and they shot these events just fine. if i were to shoot a wedding, i’d span the day’s events over more cards so as not to risk losing too much if a card failed.