My DD11 had a DS lite for a few years and just received a DSi for Christmas. She absolutely loves it and seems to spend more time using the non gaming features (camera, voice recording, internet browsing etc). We haven't got around to it yet, but we are going to add music files to the SD memory card (you'll have to buy that separately - minimum of 2G)
An acquaintance offered to buy the DS but DD refuses to sell the DS because (a) she can lend it to her little cousin when he comes over (b) she still enjoys some of the Game Boy Advance games and (c) she won't be able to use the Guitar Hero add-on on the DSi and she loves Guitar Hero.
Unless your daughter has GBA games that she plays regularly, I would recommend spending the extra $$ for the "upgrade" to the DSi. Even if she doesn't use all the features now, she will probably grow into them soon.
Actually, I think the DSi is a fantastic travel gadget for kids - camera, game system and music player all rolled into one easy-to-pack device! If you are a minimalist packer like me, you'll appreciate the fact that you'll only have to pack one charger!