My children both have a Leapster and a DS. There are so many educational games for both systems! A DS gives you more options, though. At about age 5, my son lost interest in the Leapster, but a DS brought back the interest in games--there are some that help with reading, some that help with math, and some that are just plain fun! Most games that are rated E (for everyone) have helped with critical thinking skills and have encouraged him to read more and to explore options. My almost 4 year old will still play the Leapster from time to time, but with better graphics and more to choose from an older (used from Game Stop) DS works great for him...:)
As for the attitudes of others-- I grew up playing games and now have 3 doctorates, and very skilled social skills. As a matter of fact, children find more social opportunities if they have things in common with other kids. That being said, it is common to see children with portable gaming devices. This sparks conversations, and has even helped my children find friends! :) If you look up the research studies on gaming, you will find that they show improved critical thinking skills, and better problem solving skills.
Not to mention happier parents because having a DS can help with car trips, grocery shopping, doctor/Dentist appointments, haircuts and even those moments when you just need a bit of peace and quiet.
This is a personal family choice, but may I suggest that you put your feelings aside and see what she wants? If this is something that you know will make her happy than perhaps this is a good thing--especially if she has friends who also own gaming systems. I would keep her feelings in mind when making the final decision...and yes, they run about $120-$160 each. Amazon is the best place to get games, for the prices are less expensive.
And you can always check out the games ahead of time online...there are always lots of reviews to read! :)