Hey turbot,
Thanks for contacting National Instruments support. Each set of the vis you mention are the latest, and some use similar methods to do the FFT than the other. The SMT is usually the best in most applications. There is also another option in the case of scope, you can fetch FFT data from the driver. The FFT is not actually implemented in hardware, but in the driver, and this is transparent to the user. It's simpler to use, but not as efficient as the SMT.
I hop this answers your questions. Let me know if I can be of further help.
Regards,
Kenn North
Principal Product Manager - Search, Digital Analytics
http://ni.com/search