I have never been through SEA, so I cannot comment on the airport experience there. MSP is a large airport, it can be hectic getting around there but they generally have decent places to eat and its a DL hub (so plenty of skyclub lounge access if that is your thing). I have not really had any issues with long security lines there or TSA being prickley, but you never really know when and where that will strike. I never check luggage so I am not even sure what the rule would be for that ( I think you can check it through on a domestic leg...I know you cant on international).

As for the service, my experience having flown plenty of DL and AA domestic in F and Y alike is, it is really dependent on the crew and the size of the plane as to what level of service you will experience. And really, it is mostly the crew. If you get a good attentive crew it can be good on either airline, and on any size plane. if you get a sour worn down one it will be miserable and you will wonder why you paid for F. Its total luck of the draw there.

As for the plane size, I do not know if there is lie flat on offer from MSP to ANC, I kind of doubt it (other than transcon I dont think either DL or AA uses lie flat for domestics). That might be something to look into. I would check what planes are being used on those routes at a minimum. Other than that....absent frequent flyer status I really dont see much of a difference between the 2 carriers. Maybe other members have a different view who fly that particular route? Safe travels.