Let the 12yo choose... it's not as though there's a bad choice!
But, if it were me, I might actually pick DHS. I know there's (a lot) less to do overall, but it still has some of the best individual rides (Tower of Terror, Toy Story, Star Tours, and RnRC are all in my "top 10 at Disney World" set). Shows are good (even the Frozen one - it was WAY better than I imagined). If your 12 year old is a Star Wars fan, you can see the Star Wars show at night, which will probably be new for you all.
You could spring for a hopper pass, and do a couple of parks (maybe MK and Epcot seeing that they are connected by monorail, so you might not waste as much time traveling).