I cannot imagine starting a Disney vacation without a visit to the Magic Kingdom. That is just my bias. I also have little kids that would be dissappointed if they didn't see the castle first thing - okay, I would be just as sad. I do think you need two days at least at here, so if you can get some of the park done, it will take the pressure off your other day.
If the crowd is older, going to epcot might be great. But remember many of the countries cultural performances end by about 6:00 pm, so you are going to miss many of these, which are my favorite thing about Epcot.
Don't worry about Animal Kingdom, it closes at 5.
MGM would be my husbands pick. We love the restaurants there and fatasmic is fabulous. You could still catch a few shows, have a fantastic meal and finish with a great performance.
Also, depending on your vacation length and age of kids, is there a case to be made to enjoy your resort, take a swim and turn in early? Just a thought!
Have a great trip!