Cascade impedances and Return Loss
I guess you have

75 Ohm connector -- 50 Ohm PCB connector -- Zo Ohm PCB trace -- 50 Ohm PCB connector -- 75 Ohm connector -- 75 Ohm load

It really depends on your center frequency. You should clarify this. 
However, assuming 
a) that your working frequency is low enough so that the connector length is negligible (let's say up to some 100's of MHz)
b) AND that the PCB trace is longer than the connectors (longer than some cm)
then it would make sense to make the PCB trace 75 Ohm. The small sections of 50 Ohm transmission line would give small reflections.