Prawns is better than shrimps as prawns have higher protein content and much bigger in size too. However it would depend on the species of prawn. Most aro keeper that feed prawn mainly feed black tiger prawn. Don't buy the BC prawn as they are expensive and small in size. The head just make them look bigger which you don't really need.
Many people distinguish between shrimp and prawns on the basis of size. “Prawns” are considered to be larger, while shrimp are smaller. In terms of biology, however, things get a bit more complex, since shrimp and prawns are in different suborders, indicating key biological differences between the two. Prawns are in the suborder Dendobranchiata, while shrimp are classified as Pleocyemata