This is a great question that needs a bit of analysis before beginning.
You first need to define the issue at hand.
If it is an issue that the US Congress has clear authority over, then the Supremacy Clause is going to kick in and the Federal law is going to prevail.
If it is a matter than the Federal government does not have purview over, then the 10th Amendment kicks in and reserves the power to the states and the State law would prevail.