I believe the holy word (and it's unholy side) do not give a save for the banish effect.
It's 'if you are on your home plane any non good (or evil) extraplaner creatures are banished' - all other effects are save or suck (up to dead but my players are high enough to avoid that effect). If I'm wrong about how that spell works I will need to rethink things for sure.
"The banishment effect allows a Will save (at a –4 penalty) to negate."
Hrmmm - that'll be interesting then. I may have to have a geas ready to use if anyone manages to save. I could always mind fog the area but with my luck half the party will roll 20's on the save anyway. Good news is they will be paralysed even if they make the save so if I have 'alternate plans' ready it gives me a reason to not destroy them outright and keep the story moving.
So - regarding the entire spell then - one will save - and even if they save against the 'suck' parts the same save at -4 for banishment? Or would you make them roll separate saves?