We use mosaic tile because the shower floor is shaped to slope towards the drain. With mosaics you have flexibility to fit the shape. A smooth surface wouldn't channel the water the same way. To compare the price of concrete to tile would require an estimate from the person who would do the concrete, and you'd need to compare it to the cost of the tile you like and the cost of installing it. This is such a small area, the difference may not be too much. The decision should be based on what you prefer aesthetically to complement your bathroom. If you are tiling the walls of the shower and the floor of the bathroom, it probably would look best to tile the shower floor too. If you are making concrete a feature in your bathroom, that's your decision.