Hi abdou8_x!
Normally, PMOS-pair and NMOS-pair are the same, maybe diff.pair used a lot in error amplifier and opamp...=> it depends on the output stage of your circuit. Also, it depends on your specification design. For example: Gm, slew rate, output swing, phase margin, gain margin, DC gain, bandwidth, CMRR, PSRR...by hand calculation, you can see the different btw for example OPAMP using PMOS-pair and NMOS-pair to get each parameter above.
To understand more and get more clear, i think you should simulate your problems a lot.
Hope this help!