The market determines which is best, so.............
Hardly, given sufficient suppliy the market is the result of demand (= what the people buy), not what is best quality item for a price because there are a lot of other factors at play, even in a transparent market - which it usually isn't thanks to marketing :-o
For dslrs that would be existing equipment (learning loss and loss of selling and re-buying lenses), brand attachment and 3rd party enhancements, namely Magic Lantern. Add all these and Canon can very well sell an inferior product at a higher price while Nikon simply selling cheaper might not do them any good. There is also the "snob effect" which means a higher product produces more demand because it has to be better, right :-p
I know this is getting a little off topic, but to continue:
From my own personal point of view I came across more D600 adverts than I did 6D, but I think it is really quite simple. To the average Joe who wants to upgrade to FF and doesn't have commitment to a system the 6D is simply more aesthetically pleasing than the D600/610. It's as simple as that. Also Nikon seem to fail to understand that fore mentioned Joe sees a camera without
pop up flash as much more 'professional' than one with it.
I've mentioned this before but I'll say it again. If I were Mr Nikon I would get hold of Mr Canon's chief EOS designer, stick a big wad of cash in this hand and say "HELP".