MMIC OR tRANSISTOR BASED?

I am designing s band power amplifier for the satellite on board transmitter.Can any one guide me that which approach is good either to design through MMIc or should use transistor chip e.g GaAs FET etc.

Using discrete transistors you can get lower noise figure but narrow bandwidth.
MMIC offer constant gain over wider bandwidth but higher noise figure.
Because your case is a transmitter maybe noise figure is not so important.

You also need to know the radiation dose of your flight path. Some MMIC or FET processes have not been verified as radiation tolerant. Some are such as Triquint HFET.