Drive + Park
Better if you want total control and do not mind handling the arrival, parking, and return on your own.
Argo decision page
Argo parking vs shuttle: which is actually better?
Shuttle
Better if you want the day simplified. It removes the parking question and gives you a clean round-trip plan from Denver.
Most people asking this are really asking whether they should solve transportation before they leave Denver. If that is the goal, shuttle is the better answer. Parking is only the better move if you actively want to own the logistics yourself.