Dextrose = a form of glucose, aka d-glucose or corn sugar.
Sucrose = a combo of glucose and fructose, aka table sugar.
The primary difference is between these sugars have to do with the way they are metabolized. Of the two, dextrose is said to be slightly more easily metabolized by yeast. In either case, when brewing beer, you want the yeast to go after the maltose and maltriose first and foremost. To accomplish this when using simple sugars as a portion of your fermentables, 1- limit the amount of simple sugars used in brewing, and 2- (optional) pitch your sanitized/cooled simple syrup at high krausen, when the yeast are in full effect of actively munching on the malt sugars. As always, fermentation temperature and yeast health/cell count are of the utmost importance.