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I would say get the DS too for 3 reasons. One, they won't outgrow it; you can just buy new games if you have to. Two, the DS is a popular system overall so it has a ton of games and you will have a good variety of stuff to buy.
The third reason is not very important from an educational standpoint; more from a social standpoint. A lot of their friends are going to have the bigger, more popular gaming systems. Since the older one is already 10, it's not going to be "cool" for very long to have a gaming system that is obviously one of the educational ones. Their peers are going to be playing things from Nintendo, PS, Microsoft (Xbox), etc. If you get them a DS you can still control the games they play, but they will also be able to relate to their peers. I've done a lot of volunteering at elementary schools and video games are a big topic among kids. A lot of the quiz games and things on DS are popular, so your kid can talk about their high scores on Brain Age for DS, instead of being the kid getting teased for playing something like "Reading Adventures" on a gaming system that no other 10 year old will have.