S-parameters vs device model
S parameters are measurements at the frequencies and bias conditions specified. If your design if for those frequencies, and for those bias conditions, use the S parameters, interpolating between the frequencies as required. Usually S parameters are measurements for the finished device, including all the package parasitics.
Device models are usually good at DC, for a large range of bias conditions, and reasonable at higher frequencies. If you want to bias the device, check its stability at low frequencies not covered by the S parameters, then use the device model. The model will often be for the die, I have frequently seen device data sheets saying 'if using above 100 MHz, include the following model for the package'.
Note that both model and S parameters are typically valid for the average device. Sometimes the measurements are marked as being valid for devices that are min or max on some parameter, or the models will have a spread of one or more of their parameters. You can therefore expect neither to be match completely any device you use. If the model has been extracted from a different batch of devices to the S parameters, you would expect some differences.