Did all my PhD work on an FSWP26.
The FSPN looks like a phase noise only version of the FSWP (No SA, noise figure, pulse, IQ, protocols, ...).
How much do they ask for an FSPN ? The FSWP is a lot above the Keysight pricewise, but it can also do a lot more.
IMHO, the main criteria is frequency range. If you need <10MHz (I did), Keysight has no offer, If you need < 200MHz, the Microsemi part could be an option (53100A). Above, its R&S or Keysight.
The RS parts are "integrated" solutions with internal references. These can limit performance, but a dual external reference system is not supported AFAIK. Keysight the same, Microsemi *must* use external references, making very low PN measurements possible (< -130dBc@1Hz).
What do you intend to measure ?