Dh and I don't have kids and I would do the extra day at Universal. In fact, I would buy hoppers and do it this way:
Day 1:MK
Day 2: Epcot
Day 3: The water parks and MK at night. Fun and relaxing way to recover from the parks
Day 4: Split of animal kingdom and DHS
Day 5: Universal with 1 day 2 park tickets-spending the night the night before for the express pass, or buy the limited express pass.
Day 6: free day to pick a couple of parks or a park and DTD
If you have never been, you may find yourself surprised by what you like best. In theory, DH and I are the prefect fit for spending hours walking around Epcot. He enjoys drinking around the world, we both love to shop, we enjoy the music and we both enjoy people-watching. However, we find epcot pretty boring. Instead, we fell in love with the waterparks (and now always spend at least one day), floating, swimming and having some drinks and we both love the MK. Dh likes AK and I like DHS. The hopper will give you the freedom for movement, in case you end up loving one park and disliking another.
I know many adults enjoy the characters, and parades but we couldn't care less. Between fastpass+, showing up early, riding during parades and staying after the fireworks, and skipping the true "kiddie" rides, we knock out everything we want to see easily.