Personally, I'd say it's not worth it to get a whole new system where you have to get all new games. It's not that much different. Personally, we have the leappad (it was a gift - we wouldn't have bought it but she does like it), the fisher price iXL (she loves this and it gets played with often), a Kindle Fire (great for books and Android games), a DS (I agree - not educational in the least), an Android tablet (Axiom is the brand I think). We aren't Apple people - we're Google/Android people so I can't comment on the iPad. If you're going to get something, I'd go with a cheapy tablet since you can always download new games/books/apps for it with little effort and on Android, lots of them are free. Also, check out something called "Zoodles". It's a program for the computer that takes them to various kid websites with learning games & such. It tracks what they do and how much time they spend on what for you so you can see their progress. It's pretty neat and my daughter LOVES it. :)
Edit: If you need somewhere to start looking at tablets (there are thousands of them!), I have heard good things about this one: http://www.amazon.com/Fuhu-NABI-NABI2-NV7A-7-Inch-Tablet/...