The first time you do it, don't reduce it with sugar or any sweetener because different people have different tolerance levels of what is and isn't sweet. The balsamic will thicken without added sugar and you can always add it in later if you find that the reduction is still too tart for your tastes. With that being said, I also agree that honey compliments balsamic better than sugar, and you can add that in at the end. You're going to have to reduce it by more than half though. Usually the yield is about 1/3 to 1/4 of the original quantity.
"Boldness has genius, power, and magic in it."
Johann Wolfgang Von Goethe