Both programs also let you enter additional models as user-defined equations. SigmaPlot offers more options here, although few people will ever need the additional functions that it offers. You only need to enter a user-defined equation once into either program, as you can later pick it from a list. Prism extends nonlinear regression in ways that SigmaPlot does not. These useful calculations are not built into SigmaPlot.
Prism can fit a family of curves at once.
You don't have to repeat the commands for each data set, as you must do with SigmaPlot. After fitting a curve, you will often want to use it as a standard curve to interpolate unknown values. With Prism, this is extremely easy. Your interpolated values are reported on a nicely organized results sheet. Interpolating unknown values is much more difficult with SigmaPlot.