Hey,
A device connected through fibers can be miles away, on the other hand the maximum cable distance for 10Gbase-T is just 30-50 meters. 10Gbase-T can only serve on a real TOR switch where all devices are servers and these servers are located in the same rack where the switch is. If you are in a public datacenter facility and need a 10Gbase-T connection several racks away this may not happen due to the fact that the distance is pretty high or simply because more and more datacenters deny copper lines nowadays. Also, an optic line can interconnect all kinds of equipment while the 10Gbase-T would require you to have copper ports on both ends OR to use a copper to fiber converter on the fiber port of the fiber end.
Nexus 9000 series are a very feature-rich and robust hardware but you have to consider your own needs in order to select the most proper device for you. It depends on you to clarify if you would be mostly using copper infrastructure or fiber ports, then you need to decide upon the number of ports required and uplink ports if any. This would help you understand what kind of device to deploy.
