I don't know for certain, but my guess is that there is not a significant difference in nutrient content . . . largely because there are not many nutrients in maple syrup from the start (that I do not for certain).
Superficially, the difference between the grades is that grade B is darker and has slightly more flavor. Mechanically, grade A syrup is made from the first sap taken from the tree; grade B is made from later harvests.
