Hello, I have foam glass gravel under my floor slab. I once dealt with the gravel during the construction of an industrial hall. My garage can be driven on with heavy machinery, but you can't directly feel warm feet.
I also chose the foam glass gravel. It is purely logical that the insulation under the slab is better. It surrounds the house 1m beyond the exterior walls. This way, the walls are already protected from rising moisture from the ground. You can still insulate above the slab if needed, or not, depending on the requirements. But I will never get under the house again. The small additional cost in relation to the house no longer matters.