Transmitter is transmits the signal it receives from actual sensor then transducer then to current(4-20mA) which is sent to lcal junction box-DCS- and which is again converted into the range and you will see actual value on DCS.
Transducer is one which converts actual signal (in pressures) and give output in current.
I think what you intended to ask is whather to use transmitter or pressure switch.If you intend to have trip based on this value, have pressure switch. If you wish to just control the suction pressure( in case you can run dry for few seconds/minutes), have transmitter.
If I am not mistaken, the response time of switch is less than transmitter, if both are to trip the pump. That's the reason pressure swtiches are preferred.