otti, water will hold more heat than soil, per equal volume, water is more heat conductive, and water will have convection currents. This will allow your heat pump to extract a given amount of heat from that volume more quickly than it could from soil. After a critical amount of heat has been extracted, however, the rate of extraction will be limited by the rate at which the volume can absorb heat from the surrounding soil.
The only advantage I see is if you have a need for lots of heat intermittently. I'd expect a radiant floor with high mass to have a fairly constant demand for heat, based on the rate at which floor is losing heat. Perhaps in a passive solar setup, where the slab is exposed to high amounts of sun during the day, and needs heat only at night.. then the water could absorb heat during the day and release it at night. But freezing of the water is a concern - it would need to be sized appropriately to meet the load, and you'd probably want a temperature sensor to prevent the heat pump from freezing the water under extreme loads.
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