I love MGM/Hollywood Studios - in fact, it may be my favorite park. Epcot is not my favorite, but I still enjoy it; depends on what kind of person/family/group you are.
Do you like shows? Not a lot of walking? Sit-down old fashioned American food and a taste of Hollywood?
I could spend days at MGM - between scheduling in the 25 min. or longer shows: Little Mermaid, Beauty and the Beast, Muppets, PlayHouseDisney, Indiana Jones, Highschool Musical, Lights/Motor/Action Stunt Show, and Fantasmic...and seeing the rest of the rides (tend to be longer, half-hour rides) such as Catastrophe Canyon, Great Movie Ride, Animation Tour, Star Tours, PLUS waiting in line for Tower of Terror and RRC...I could do two days easy at MGM. We love the restaurants there as well.
Do you like lots of walking? Drinking? Bands? Shopping? Scenery? Pricey but delicious non-American cuisine?
Then do Epcot - my parents could drink and walk around all day there and watch bands and do wine tastings and do some relaxing rides and shopping.
Future World of Epcot could keep kids occupied for half the day - the countries hold little appeal for restless youth (in my opinion.) At least at MGM they'd be sitting in shows, a few of which are air conditioned...
Have fun whatever you choose!
LOL...I just re-read my post and I sound like a Disney commercial...sorry about that!