Travelling between SAI and KTI via plane.
Umpteen times more expensive than a bus, and that's just the airfare itself, without adding in transfers to and from the respective airports, then factor in all the time. There are no lounges and you get shunted into the butt end of both airports (since Cambodia doesn't have much to go on in terms of domestic operations, naturally they don't need many check-in desks, security checkpoints or gates, hence no lounges and few concessions). Onboard got a bottle of water and a biscuit (similar to a shortbread).
While I have no direct experience, if I had done a bit of research apparently there are buses between both cities that have aircon, wifi and even snacks included.