"classical radar motion detection " ==> classical motion techniques rely on the doppler information inherent on the carrier wave. Now, to fetch the doppler information you need to make abservation, or in other words check the time derivative of phase at a specific frequency. So you need a series of phase readings from the network analyzer.
"s parameters are ratio-ed numbers", this ismuch better, since radars also rely on the ratios of transmitted and received carrier.Usage of reflectionor transmission parameters rely on your application. Try to focus on the case with higher SNR!
In motion detection phase is much more important than amplitude but as asserted before, you need better than enough SNR to correctly quantify the phase. Amplitude data can be utilized with phase info for heuristic motion detection algorithms.