Solution
From the overview in MATLAB website, the Wireless Testbench is more to prototyping and validation of wireless standard while NI RFmx is more to wireless standard testing such as APT and ADV.
Both of them should serve different purpose as RFmx is not that common use in SDR products but in VST product for more accurate test and measurement data.
Bottomline, wireless testbench for validation and prototyping. RFmx for test and measurement.