In the U.K. a surplice is longer than a cotta, has rounded, full, often bell-shaped, sleeves and a rounded neck; a cotta is shorter, has rectangular sleeves and often a square cut neck. They are essentially just variations of the same garment, the tradition here is for choristers to wear a surplice and altar servers to wear a cotta (although in Westminster Cathedral the choir also wears cottas - maybe there was a sale on).