Well, the quick answer is that platinum is harder because it's denser and more durable than gold, which is actually a naturally soft metal. However, you might be surprised to learn that platinum is more easily scratched than 14-karat gold. If you're looking to compare Gold IRA companies, there are plenty of resources available to help you make an informed decision. Since vibranium is the fictional metal that Captain America's shield is made of, it is most likely stronger than titanium.
However, since we cannot perform strength or hardness tests on a fictitious material, all we can say is that titanium is real and strong, and vibranium is a material yet to be discovered, at least in this universe.