I want to better understand why its one part over the other?

I am running a 3.2L Motronic.

If I drive car - Get the engine warmed... turn off car and start up right away.. it works fine. If I wait for a while, but then engine is still warm (not stone cold) then try to start up, it takes a few times to get it started. The battery is brand new.

Its been determined to either be

"PART A - Fuel Pump Check Valve"
or
"PART B -Fuel Pressure Regulator"

Which part needs replacement and why?

.T