Solution
From the overview in the MathWorks website, the Wireless Testbench is more focused on the prototyping and validation of wireless standard while NI RFmx is intended more for wireless standard testing during automated production test or automated validation.
Both products should serve different purposes, as RFmx is not as commonly used with NI SDR products but it is used with the NI PXI Vector Signal Transceiver for more accurate test and measurement data.
Bottom line, consider the Wireless Network Toolbox for validation and prototyping, and RFmx for test and measurement.