The difference between sweet and tart cherries is not so much the sweet, but more the tart. If you look at the nutrition information you'll find that tart cherries have about 2/3 as much sugar as sweet, but twice as much Vitamin C (ascorbic acid) which is what gives them their tart flavor.
With this in mind, most sweet cherry pie recipes include lemon juice, while tart cherry pies recipes don't need the lemon juice for balance.
Recipes for both type seem to use between 1/2 to 1 1/2 cups of sugar, so as Stephie says, "It's mostly a matter of taste"
From personal experience - I made 2 cherry pies using a tart cherry pie recipe, except substituting Bing cherries. I got more great reviews for those pies than you would believe. (It might have something to do with the sweet tooth gene on that side of the family)