DH and I are in our late 30;s and we love Epcot. We don't spend more than three hours in MGM and I can't imagine us ever spending more than five there. It's a half-day park. There's no really great restaurant in MGM, and most of the shows we like we go to because of our 3 and 4 year old kids (Voyage of the Little Mermaid, Beauty and the Beast, Muppet 3D movie). I'm not a thrill ride person, so I skip Tower of Terror and Rock and Roll Roller Coaster.
At Epcot, you can check out the Future World stuff, like Living with the Land, Soarin', Test Track and Mission Space. Then you can spend leisurely time wandering through the World Pavillion. There are great dining and snack options all around. We also like the Chinese acrobats, the storytelling in the Africa section with drums, the musicians in Germany and the concerts in the UK pavillion. We usually do two loops around when we're there. It's romantic at night as well. And then you have Illuminations.
If you could only do one, I would choose Epcot. If you can swing two in a day with a hopper pass, go to MGM in the morning and then go to Epcot for the rest of the day.