D.D.
We have a 3DS, and both my kids - age 3 and 7 - love to play with it. You can use all DS games with the 3DS (but not vice versa), and if you get 2, the kids can play the games against each other. Neither likes the Leapster, it sits on the shelf unused (to be fair, they did like it until my older child got a 3DS, then it instantly became uncool and not worth the time, even for my 3 year old).
The only other thing I'd consider is an iTouch, because they like playing the games on my iPhone so much. And games are so much cheaper than DS games. Plus they can stream kids' shows, facetime with each other (and with cousins if anyone in their house has iPhone/iTouch/iPad).
Whatever you decide, I'd get the same for both. Otherwise, they will just decide one is better than the other, and they'll always be fighting over that one.