I would suggest Epcot. In addition to being the most pleasant park when crowded (IMO), it has several popular rides: Test Track, Mission Space and Soarin. TT and Soarin are Sarah's 2 favorite rides at WDW. There are also several tamer rides that would be good for your younger DS as well.
HS has 2 major rides, RockNRollercoaster and Tower of Terror. Don't know how you feel about those, but both are too intense for my thrillseeker. Other than that there are several shows there, so if your kids are not into shows it isn't such a good park.
I do like AK very much, but I do not like it when it is crowded. It's a difficult park to navigate, IMO, when crowded.
Beth, mom to older DD (8/01) and younger DD (10/06) and always missing Leah (4/22 - 5/1/05)