IMO, its a personal decision. Some people don't care for the lack of rides at Epcot. Some people can spend hours taking in the details, movies, and stores around the World Showcase. Some folks with kids don't care for Epcot because it doesn't keep the kids' interest. It all depends.
For us, we spend about a 1/2 day or slightly longer doing all the Future World rides. Another 1/2 day in the World Showcase. And Epcot is our favorite park. Personally, I think either way you go will be fine, but the two 1/2 days might be best so you don't feel the need to kill time doing a full 12+ hour day at Epcot.