If you could spare 2 days, you should do both. If she has never experienced MK at all, they go there, unless you are going at a really busy time on a busy day.
Both are great for that age child, and I am sure she would love the animals at AK and the shows. Honestly, I wish there were more rides for little ones, as really, the only ones she can go on are Triceratops Spin, the Train ride to Rafiki's Planet Watch and Kilomanjaro Safari.
At Magic Kingdom, there is Peter Pan, Small World, Dumbo, Snow White (may be a bit much for her), Winnie the Pooh, Buzz Lightyear, Astro Orbiter, Jungle Cruise, Pirates of the Caribbean, Aladdin's Magic Carpets, the WDW train, Cinderella's Golden Carousel, Goofy's Barnstormer, Mickey's Philharmagic, Cindercelabration, plus Country Bear Jamboree, Tom Sawyer's Island, plus all of the opportunities for pics with the characters, the parades, plus Wishes at the end of the day.
Of the 2, if you want to make the best memories for her... she will likely remember MK more than AK. If you can do both, that would be best!