It is important to note that osmotic pressure is a colligative property, which means that it does not depend on the identity of the solutes, but simply their concentration. This also means that in considering the grape, we have to estimate the concentration of all solutes, not just the one you used to make the (possibly) hypertonic solution.
I'll assume the sugar used was sucrose, which is the common table sugar. I guess I should also mention that I'll assume the salt was NaCl, which is the common table salt.
Wikipedia says that the solubility of sucrose is 2000 g/L. As the molar mass is 342.3 g/mol, the concentration is approximately 5.8 M.
NaCl on the other hand, has a solubility of 359 g/L. At 58.44 g/mol, the concentration is approximately 6.1 M. However, the Na+ and the Cl- dissociate in water, so the concentration is effectively double when counting solutes. That gives 12.2 M, which is much higher than a sugar solution. This accounts for why grapes would shrivel in a salt solution more than in a sugar solution.
As to why the grapes did not shrink in a sugar solution - I don't think it is unreasonable for a grape to have total solute concentration in the range of 5.8 M. Estimates of the sugar concentration in a grape are given in % w/w. Clearly grapes are denser than water (you can test this by putting a grape in water and seeing it sink). Therefore if you were to calculate in % w/v, you would find it to be more concentrated than a the same % w/w solution in water. Moreover, according to this site grapes have a significant amount of glucose and fructose. These are monosaccharides, as opposed to sucrose which is a disaccharide. This means for the same mass of "sugars" there are more individual sugar molecules, resulting in more osmotic pressure.
In sum, I think the sugar solution is probably isotonic, not hypertonic.