I use stevia (Target has a generic version(Market pantry)), and for us, it takes 2 cartons (9.8 oz each) of the stevia to sweeten a 5 gallon keg of root beer for our kids. I force carbonate, so not an issue of fermenting.
Since it is the sugar that usually contributes to weight gain among carbonated drinks then I guess it would be a good idea if you will use artificial sweeteners like aspartame, saccharin, cyclamates, sucralose and/or acesulfame potassium.