Actually both fish belong to the same Sparidae family but, apart from the fact that it's difficult to see large gilthead seabream (about 3.5 pounds or 1.5 kilos) like this one at 5 meters depth (they can move in depths of up to 150 meters), I had never seen one traveling among a large shoal of sand steenbras.
The main differences between these fish are:
Gilthead seabream (Sparus aurata) feeds on other fish and also crustaceans or shellfish. It's highly appreciated in gastronomy and its prices can be around € 8/kilo in every fish market of the world.
Sand steenbras (Lithognathus mormyrus) feeds mainly on sea worms, although its feeding spectrum is very wide, including sea urchins and molluscs. It's difficult to find them in the fish market but if you fish some with a rod, their meat is soft and good to eat.
You can find both fish in the Mediterranean Sea, as was my case.
There was poor sea visibility due to the heavy swell.