The Airbus pedals have a built-in rest for the heels, when they are up. Also, the rubber covering makes it more difficult to slide the feet up. For these reasons, I prefer to be feet up in the ‘bus.
On the Boeing, there is no heel-rest and the pedals are shiny, which makes it easier to slide the feet up. For these reasons, I prefer heels on the floor in a Boeing.
I too have experienced a student inadvertently applying brake pressure during take-off. Difficult to prove, as it was in the aircraft (Citation) and therefore no read-out but I could feel some snatching during the roll and a definite lurch into the air when the wheels left the ground.....