Corn sugar (Dextrose) is what you want, especially if you are intending to get close to the alcohol limit for your yeast. Dextrose is more easily used by yeast than table sugar (cane sugar).
Table sugar can be "inverted" to break it into Dextrose and Fructose before adding to your juice.
When I am boosting an Island Mist kit (a low alcohol kit), I use straight table sugar, as it is getting no where close to the alcohol tolerance of the yeast.