Welcome to the DIS!!! Sunday is scheduled to be an evening EMH (Extra Magic Hours) day at MGM for Disney resort guests, so I would avoid that day. It will probably be crowded in the morning too because some people staying onsite will have park tickets that do not include hopping.
Friday is actually morning EMH at Magic Kingdom, so that should be a great time to go to MGM, especially if you can get there at opening. Most of the resort guests will be a Magic Kingdom that morning.
Let us know if you have any other questions. Make sure you use Fast passes as much as possible, and check out the new Stunt show ride. If you want to take a mid-day or dinner break, you can take the boat to the Boardwalk or Beach Club Resort. There's several fun restaurants at the Boardwalk--the Brewery and ESPN Zone, and Beaches and Cream at the Beach Club is the best place to get burgers and ice cream. It does get crowded though.
Have fun!! Ride the Rock N' Roller Coaster and Tower of Terror for me!