I’m going to dissent and say to not write-off going to AK for a day. You’ll get two completely different experiences. But it really is up to you.
I just took a 3 & 4yo and honestly, the AK day was far less structured and this was a good thing for my kids. Kids that age want and need to run around and explore things on their own. They like to walk around and play with things, and “be on their own” albeit within reason at a theme park. MK is all about rides, attractions, and lines. Sitting down and waiting. Standing up and waiting. My kids tired of that routine real quick. It was get in a stroller, get-out, get in line, sit down and ride or watch. There is a lot of cool stuff to see and do at MK, I just noticed there isn’t a lot of places where the kids can get out of the stroller or not stand in line and just “play” or run around or just be free to be kids. There are exceptions (i.e. Tom Sawyer, Storybook). In fact, my kids had so much fun in the playhouse under the train bridge next to Splash Mountain.
AK, on the other hand, has tons of places for this. The animal trails, The Dig, Rafiki’s planet watch. The kids were much happier playing, walking, and interacting with things on their own.
What I’ve learned over my trips to Disney is why am I there. Am I there to do what the kids want to do or what I want to do (which is see everything)? And I’ve learned to really watch what makes them happy. And my kids really did prefer The Dig over standing in line for some ride.