Greetings Rich,
Using the SCXI channel strings will actually read back the excitation voltages using the A/D of the DAQ device cabled to the SCXI chassis. Using the AI Paramenter VI to get the excitation value will just read the NI-DAQ configuration file and return what you set the excitation value to within MAX or LabVIEW. So the AI Parameter VI should just be used to ensure that the excitation was set (this isn't actually reading the excitation voltages using the A/D of the DAQ device).
So, reading with the SCXI channel strings will allow you to confirm the voltage levels of excitation. Using the AI Parameter VI will ensure that the setting for excitation has taken affect.
I hope this helps. Let me know if you have additional questions. Have a great day.
Regards,
Todd D.
NI Applications Engineer