Depending on where you live, you'd probably get more bang for the buck with an air/water heatpump. Rather than just getting a watt for a watt of heat, you might get as much as 4-5W for one watt input. Depending on your electric rates, that could be huge. With radiant slab, it can take days to warm it up, but once there, it's pretty stable. Much differnet feeling than warming just the air.