The metamorphic rock went under heat and pressure before being sandwiched in silt to become part of sedimentary rock. Think of it like the McFlurry. You've got to bake the Oreo before adding it to the ice cream.
Answer: The metamorphic rock would be older, because in the sedimentary rock's formation that is caused by collected sediment taking years to harden and turning into a rock, the metamorphic rock would have already been considered metamorphic when traces of it came into the sedimentary.
