Corn dextrin is truly gluten-free. Wheat dextrin despite claims that it is gluten-free, still contains 20 parts per million of gluten and is still suggested to be avoided by people with gluten allergies. Corn dextrin has a lower glycemic index which means less turns into sugar in the body.
Corn Dextrin occurs naturally, so it only requires hydrolysis to separate the soluble fiber from the rest of the corn.
Wheat dextrin is considered semisynthetic because it has to turn the wheat from a starch into a soluble fiber. It is also not possible for the dextrinization process to fully convert the wheat, so some is still left as starch which cannot be digested and immediately goes to the small bowel and turns into sugar.