Welcome.
Both great packs. The marshal is a beast. Most people would only need a pack that big for +3-4 days. The Metcalf is probably the most versatile pack MR has and sized for 3-5 days for most people.
IMO the difference between 3 days and 4 or more is mainly consumables. I might bring a bit more extra gear and clothing for a trip that long but after I pack all the shelter, sleeping and cooking gear for 2-3 days, staying longer is food, fuel and some misc other stuff. With that in mind, some prefer to buy a smaller pack and then add extra capacity via the meat shelf or by strapping stuff to the outside that is consumed during the trip. By the end, you have used up enough food, etc. that everything fits in the pack.
With that in mind, I would ask yourself what types of trips you will do in the future. If you will do mostly 1-3 night trips, the Metcalf is fine and can work for longer trips. If you are going to make a habit of 4, 5, 6 or more day trips then a marshal is the way to go.
For my part, I had a marshal sized pack and I was just not filling it on my shortish excursions. I used a Metcalf (now a beartooth which is the same rough size) and it works fine for my needs plus I can load it up for the rare long trip.