My DD (8)'s favorite park after MK has always been EPCOT. Tons of things to explore, tons of Princesses and other characters, Nemo, Turtle Talk with Crush, Living with the land, Soarin, the Donald Duck boat ride inside Mexico, Journey into Imagination, and more. We spent 1 day there from open to close when she was 2, and that was not enough time. Since then, we have spent 2 days there on every trip. Also at there age, they might love the Phineas and Ferb missions at EPCOT. The 4 year old is a little young, but will probably love tagging along with your 6 year old doing them. They are really fun, they get a commicator and follow clues in the countries and they actually trigger some pretty cute special effects with the communicator. DD went absolutely nuts over those missions when she was 6 (DH did too- I had to pry the communicator out of his hands to make him let DD have it!).
As far as AK, DD has never really cared much for it. No Princess presence at all except for Pochahontas. It is generally a half day park for us. Until last fall, she was always begging to go back to MK or EPCOT by noon. They did start a Wilderness Explorer badge accumulation booklet deal last fall, and at age 7, she did really enjoy that. That did keep us there a while longer. They have explorer missions they do where they learn about things and collect badges. I think the booklet has about 30 different sticker badges they can get. I think one of the reasons DD has never been really thrilled with AK, aside from the fact it has no Princess presence, is that we have a really good zoo at home. She does not want to spend her time looking at the same animals we have at home, when she could be doing "Disney" things. If your girls love animals, and you don't have a decent zoo at home, they might really love it there.
Even so, if we only had 1 1/2 days for both parks, EPCOT would always be our pick for the full day. Another thing to think about- I don't know if your 1/2 day is morning or evening, but World Showcase at EPCOT does not even open until 11:00 a.m.