Hardwood Flush Door or Pinewood (softwood) flush door, which is better?
Warp is a very important point to consider. Though depending on the species and the cut, this answer could very a lot.

Pine may be lighter, but also has a much higher moisture content.  If the door is kiln dried and sealed quickly, you can keep it from absorbing the moisture from the humid air.  Also if it is air-dried, it may be more stable. I have seen pine warp significantly due to changes in humidity, and being in India -- especially if you are in the South or East -- I would wonder if this wouldn't be the case as well.  (especially if it was a knotty pine)

Hardwoods are denser, and generally have a lower moisture content allowing them to dry well and remain more stable lowering the chance for major warp.

My opinion: Go hardwood.