Hachiya are longer and more "peach-shaped." They need to be eaten when very soft. Best eaten chilled, or scooped out with a spoon, these ones are best for cooking with.
The Hachiya is different: when it's 'fruity', firm and a bit crisp, its astringency makes it just about inedible; when it's ripe and sweet, its very soft to runny gel texture makes it seem just about inedible, because it's the texture a lot of fruit assumes when it's spoiled. This progression is quite like that of a banana. Bananas when green (that is, green or even greenish peel) have similar astringency but not to the degree of that of a firm Hachiya. As the banana ripens it loses astringency, increases in sweetness, and softens. If you let a banana get fully ripe it turns to a translucent brownish gel, so sweet that much of the fruit's subtlety is gone or overwhelmed. The Hachiya reaches true edibility at this last stage, when the flesh has turned to translucent gel. It has the advantage of the banana in that the gel is a brilliant orange, something like a dye they'd put in anti-freeze or shark repellant, but the texture is distinctly slippery; unlike the banana, a bit of astringency may remain, similar to the tannin in a red wine. You know a Hachiya is ripe when it starts to deform under its own weight and feels like a bag of jelly when you handle it.
The ripe Hachiya is quite edible on its own: you can suck the flesh out of smaller ones after biting a hole in the fairly tough skin, or you can cut them in half and use a spoon. They can also be peeled and frozen, and eaten as sherbet. There are, however, culinary contexts which their the color and texture perfectly suit them to, so we'll appeal to another aspect of intelligent design, making a virtue of necessity.