Amtrak coach seats are going to be only a few bucks higher than MARC fares, if not the same price. But MARC tickets can be used at any time of day for whatever train is most convenient for you. Amtrak is for a specific train at a specific time. If you don't know how long it'll take to get from baggage claim back to the station, I'd go with MARC and maybe Amtrak to drop her off when she leaves.