Among the major differences between the NEX-7 and the NEX-5RK would be the Effective Picture Resolution, wherein the former boasts of an approximate 24.3 megapixels whilst the latter has an approximate 16.1 megapixels. The NEX-7 also has a built in flash and an option to put an external Sony flash along with a built in viewfinder. More information on the specifications of the 2 can be found on the links provided below.
NEX-5RK - http://www.docs.sony.com/release/specs/NEX5RKS_mksp.pdf
NEX-7 - http://www.docs.sony.com/release/specs/NEX7_mksp.pdf