Considering updrading from RX100 to NEX-5N or NEX-6

One thing to point out that there is no fast zoom for NEX comparable to the fast zoom on the RX100. So if you like multiple focal lengths handy, you won't get better low light performance (despite the larger sensor) unless you carry around multiple primes with you. This is a big bummer for me personally, and reason enough to seriously consider switching to the Fuji X-E1 with the Fujinon 18-55 f2.8-4.0. Fuji, unlike Sony, realizes that a fast (and cheap!) zoom lens is necessary to make full use of the advantage of an APS-C sensor over 1-inch sensors.

That said, even with the very slow Sony 16-50 zoom the larger sensor will likely buy you improved dynamic range over the RX100.