Wax sticks to the top layer of your skin, so when the hair is ripped away, so it the top membrane layer. This leaves you more susceptible to infection, also, it fucking hurts. Sugar just sticks to your hair and leaves skin intact.
Wax uses paper strips, so much more waste. You use the same sugar for the whole appointment, no paper is used.
Wax uses a higher heat point to work, which is why so many people get burns when waxed. Sugar uses a much cooler heat point, so it won't burn you.
I just don't see any pros to waxing over sugaring. Some people claim sugaring lasts longer, but if it does, it's not enough of a noticeable difference, imo.