The SP145 would be the better choice for phase noise measurements due to it’s better absolute phase noise performance and higher frequency range.
The purpose of the preselector is to filter out of band energy. This is most commonly helpful when measuring over the air with an antenna, as communications signals such as broadcast radio, cellular, WLAN, are filtered, improving measurement performance such as dynamic range.
Phase noise measurements are commonly/ideally made directly connected to the DUT via coax, eliminating the presence of out of band energy unless generated by the DUT itself. You can always insert a filter if out of band energy was a problem for a particular measurement setup.
