It really depends on your child. MGM is our least favorite park with our 3 year old DS, because, other than the HISTK playground, it's mainly shows that are at least 20 minutes long. MGM=lots of sitting and watching. He did enjoy Playhouse Disney and MuppetVision, but didn't want anything to do with the Little Mermaid or Beauty and the Beast, and he doesn't have the patience for most of the other shows/attractions.
On the other hand, he absolutely loves Epcot. Ice Station Cool, playing with the Thomas trains in England, watching the model train in Germany (he loves trains!), making a mask and visiting all the Kidcot stations, going through all the TestTrack exhibits (loves cars, too!), watching the dancing fountain, the fiber optic sidewalk, talking water fountains, etc. Also, there are so many great places to get out of the stroller and run around, which helps burn off some energy.
I think, as adults, we tend to think of Epcot as being more adult-oriented because there isn't an abundance of character-based rides and attractions, but there are so many little things there that can be fascinating for a 3 year old to discover.
I would suggest spending the day at Epcot on the 22nd, rather than the extra morning at MK, but still head back to MK in the evening to catch Spectro and Wishes.
Good luck, and have a great trip!