It's also very likely about carrier relationships and the slots they may or may not have in their lineups. With the lower price point the C6 can be offered very cheap or free on contract, whereas the a little more expensive and polished C7 may allow the carrier (and hence Nokia) to squeeze out a bit more revenue.
Also, some may just prefer the look of one over the other of course.
As for NFC - well, yes, it may never break through but it does now have some momentum, including as other manufacturers are also starting to include it in their devices.