Dear Anjan,
It is always advisable to go in for a gypsum ceiling as compared to pop due to non combustable nature of gypsum board.But it weighs your pocket down a bit. Moreover gypsum is bigger in size as a result lesser joints are there in your ceiling. To prevent heat on the top floor you can fix 25mm thermocol sheet to ceiling before carrying out ceiling in gypsum or pop as you may like.
Regards.
Mannu Bhanot
C.E.O.
Wood Magic Interiors