^This. I initially think floorless is going to be the scariest, due to you being held by your private area, but I fear more about the brakes than the drop itself. When standing with a floor, it feels more like the sensation when you're on a hotel lift, however, only to get to the top, high up, tilted forward and the lift breaking for a free-fall. It scares the **** out of me everytime, and tends to get my tummy more than the floorless side, plus the feeling of your feet lifting off is crazy!
Without a doubt floored. Floorless is painful and, similar to furie's point, when your floorless I think you subconscienciously think you'll be okay because you're feet are below you, so if you fell you would land on your feet. I know you would die, but I think, at a subconscience level, there's something reassuring about that.
With the floored, it forces to feet to be in a position that make you feel like you're facing the floor, therefore being at an unease in that position.