Upload & Sell: Off
You left off the one difference that was probably the most important to Joel. LESS WEIGHT.
I just completed a 100 mile trek through Patagonia a few months ago and I can tell you that I would have welcomed any reduction in pack weight. Doubt Joel was actually trading camera weight for water weight, however. You can drink from any river or stream in Torres del Paine or Mount Fitz Roy parks and you find them everywhere.
Indeed, the 6D weight factor is huge, wifi, gps, interchangeable focusing screens, -3ev center point; although the 5D3 is pretty good also at -2ev, and high ISO IQ is a tad better with the 6D.
It's really a tough choice, but I'll probably get a 6D kit, the 24-105 will round out my f4L zooms, and I'll probably pickup a 1D3 for it's higher fps, killer AF and AF f8 abilities. The 1D3 being my action/wildlife body, 6D for city/landscapes, macro, etc. Who knows, by the time I take the FF plunge a 1D4 might be within reach...