In terms of technological advancement, NEX 6 really 'replaces' (notice the inverted commas in my previous post too) NEX 7, due to newer and better sensor technologies and features (wifi, apps etc... things that pros might dismiss readily but it's a technological advancement that we cannot ignore ). Do note that technological advancement is independent of whether it's top-end or not. Comparing 6 and 7 is like, a not-so-good-analogous-example, Canon 6D and 5D2. 6D is newer and better in alot of ways, but in terms of line-up, 6D is still 'entry level' and is below the 5D series. 5D2 still have certain features that betters the 6D. But the gap in line-up between 6 and 7 is so narrow, that its newer features (and price) are making people feel that it replaces the 7.
Of course, those are just spec sheet comparison. We don't have the actual cam to make comparison, but at least one famous reviewer Steve Huff have felt that his pre-production NEX 6 is better than the 7.
If you need to make large prints, of course the 7 would be better for high MP low ISO performance.