One advantage of heating water over sand is that you can heat it up with high temperature heat pumps which currently have CoPs ranging between 2.4 to 5.8 [1]. So for every kW of electrical energy you put in you get at least 2.4kW of thermal energy out.

So yes, the volume of 95C water would be much greater than that of 600C sand, but if volume wasn't an issue you could do it much more efficiently. Alternatively, you could use battery storage for just the electrical capacity required and not the (much higher) thermal capacity which may be more cost effective when you look at the conversion.

[1] https://www.sciencedirect.com/science/article/abs/pii/S03605...