I don't use either one. Both are approximations that vary with the temperature being measured.
I use this equation (or a variation thereof), from the eFunda page,
put this into an Excel worksheet or Mlab code, and compute the sensitivity for any temperature I desire.
Note that you also should consider how you're going to measure the resistance of the thermistor (current source, voltage divider, etc). In the end, you probably care about the sensitivity of the thermistor + measuring circuit, in mV per deg C, or something similar.
Added Example Below
This is an example. Thermistor is a Betatherm 10K3CG3 (10 Kohms @ 25 C), with a parallel resistor excited with a 250 uA current source. Curve is the sensitivity of the circuit.