Clear candi sugar (fructose/glucose), Sucrose (table sugar), and dextrose (corn sugar), rock candi sugar I believe is also simply sucrose in a "rock form".
I would use table sugar, it is cheaper, and you will have less of a chance of it scorching on the bottom of your kettle because it will dissolve faster.
AS far as flavor goes, they will all be very similar and should add relatively little to none. Candi syrup (inverted sugar--fructose/glucose) is said to prevent yeast from producing invertase, which basically means the yeast doesn't have to produce an extra enzyme to metabolize the sugar.
FWIW, table sugar is just fine and it is what I would recommend if you are simply trying to boost alcohol, and dry out the beer.
Darker Belgian Candi Syrups is a whole different story.
Cheers!