Hi L.!
I can't give you any input on the Leapsters, but I think the DS can be educational depending on your definition. Some games are truly educational like Brain Age - math skills, memorization, reading speed, etc. But adventure or role-playing games can also help children with problem-solving skills (items that must be found, tasks completed to move on in the game) and even empathy and morals (defending weaker people, saving the planet).
Here is an article about the benefits of video games that you might find interesting. http://abcnews.go.com/WNT/Health/Story?id=814080&page=2
Just make sure you check the age rating of the game so that the content isn't inappropriate. A few games you might like: Legend of Zelda series (adventure, role-playing) and Animal Crossing (you buy a house, have to pay off the mortgage, catch fish and bugs, decorate your house, water flowers to keep your town looking nice, etc.)