Nexus 7K and 9K are the 2 Cisco families apposite for Data Center environment with Nexus 9K being the new entrant of the 2 flavors and having an array of top-of-rack and chassis based switches.
The Nexus 9000 switches run NX-OS same as Nexus 7000 and banking on key features like ACI (Application Centric Infrastructure) for allowing the switches to participate in the SDN framework. Though Nexus 7000 series doesn’t participate in ACI, however will function as interconnect devices, linking together multiple ACI setups.
N7K being a little high on cost, provides feature set like OTV, LISP, VDC and FCOE which latter doesn’t venture into presently, still N9K has some features and roadmap which can’t be undermined. Another benefit of Nexus 9k being that it can be deployed in a standalone mode with existing Nexus switches (If customer is still exploring) and may plan to introduce Application Centric Infrastructure in near future , then Nexus 9000 would be the recommended platform.
Below are some of differences between cisco Nexus 7k and Nexus 9K which help gives more insight into platforms –