Turtle or a Tortoise?

Neither. Both turtles and tortoises require more care and husbandry than your local pet shop may let on, and are not typically suitable for children. If you really really want a turtle or tortoise then spend a couple months doing research to decide which is best for your situation. 

If the child wants to hold the pet, a tortoise is better because turtles live in the water. Taking a turtle out of the water stresses it. Tortoises however are more expensive. Therefore it may be best to get a box turtle. They look like a tortoise and they live on land but they are much cheaper. In certain parts of the USA, you can find them in the wild easily.  During or shortly after rains, they can be found on the road in rural areas. If a child is content to simply watch a pet, a turtle is more active and more interesting to watch. 

How old is the child? Neither is recommended for young children under the age of five as reptiles do pose a fairly significant risk of salmonella to children, and young children have a tendency to touch their pets and stick their fingers in their mouths without washing them well. 

For older children, you just need to look at the care of each and make a call as to which is the better fit for your home. Many species of tortoise get fairly large and require a lot of space as well as specific seasonal temperatures and a variety of daily fresh food. Turtles often can be kept in smaller enclosures but water changes and etc can take a lot of work. Neither is particularly low maintenance as a pet.

Do you want a land pet or a water pet?

Well, if you're the parent looking for easiest, go for a tortoise. Turtles need water and tanks, water cleaning and changes, and turtles notoriously have eye and shell issues. They are very fussy animals to have, in my experience. Some also require live fish to eat. A tortoise is fairly easy to own. A pen or tank, heat, and some lettuce, the end. I'd say the only issue you face with a tortoise is how long-lived they can be. Your kid may end up taking him to college, and have THEIR kids take care of it, LOL.