Dextrose is free glucose.  While a case can be made for using pure maltodextrin, I find that palatability is improved by mixing in dextrose.  Unlike maltodextrin, dextrose is sweet, albeit not as sweet as table sugar.  By adjusting the ratio of maltodextrin to dextrose, virtually any level of sweetness can be obtained.  Nutritionally, the two are identical, though some research suggests that maltodextrin is more readily absorbed.  Dextrose is somewhat cheaper than maltodextrin.  Like maltodextrin, it is most readily available at brewery supply shops.