You can use either, though if you go with molasses, use a tablespoon plus a teaspoon of molasses for two tablespoons of barley malt syrup.
According to the
Cook's Thesaurus:
barley malt syrup = barley syrup = malt syrup = dark malt syrup = malted cereal syrup = extract of malted barley Notes: This tastes a bit like molasses, and it's not as sweet as sugar or honey. It's mostly used to make beer, but it's also used to make breads or other baked goods. Substitutes: molasses (1 cup barley malt syrup = 2/3 cup molasses) OR rice bran syrup (1 cup barley malt syrup = 4/3 rice bran syrup) OR maple syrup