Each Marine has to go through the same training whether they are going to be on the front lines or not. Every enlisted man is trained to be a rifleman, every officer is trained to be a provisional infantry commander. The Marines were designed for combat alone. While the U.S. Army is very tough as a whole, the average member of the Corps has to maintain higher levels of fitness and combat readiness.
Head to head, Army might win by numbers. Man to man, Marines win every time.
We know US marines train very tough, but if we look at the documentary, surviving the cut (us rangers), they only eat one meal/day, and on the first day of the ranger schools, they end up dropping. May be the Navy Seals might go through similar training, but the us rangers like the marines are trained to fight anywhere.
It depends on what you define as tougher. Are marines individually more dangerous and more highly trained? Yes. But the U.S. Army has much greater numbers and access to a multitude of different resources and weapons. By strength of numbers, the U.S. Army would be tougher to defeat head to head based on strength in numbers alone.