my friend had a truck with a single sub and he put cinderblocks on the floor and set the box on top of those to make the sub firing at the ground but so the sound wouldnt hit anything and to give the port plenty of room to let out air and that seemed to work best for him, but of course nobody could sit in the back of the truck
back in the the day. mid 90's. I had a 18" rockford punch. extended cab s-10. I made a box with the sub facing the roof of the truck. pretty loud. Then I made one with the sub and port facing the floor. off the floor about 6" inches. Made a HUGE difference. My vote is for downfiring.
downfiring...the sound bounces off the hard surface and creates more sound...I have an 01 f-150 and i used to have 2 12" shallow mount Kenwood Exceleons downfiring under my backseat.
It all depends on your particular install. No one can tell you what will be better in your case even if they think they can. That said, downfiring often works well as long as theres some space between the subs and the floor. Sometimes in trucks theres so little space unser a seat that the subs are right up against the floor, then you lose output.