These products are all the same prices a few dollars off from one store to another - or they're sold with a different "bundle" or package of accessories. If there's a big difference - like $112 vs $149 it's probably a different generation - such as the regular dsi and the new dsi with the larger screens. In all my years of video games & ipods the manufacturer seems to set a price and the only way the store can differentiate themselves is to offer different bundles... I don't know how old your child is and wheter he'd notice - but many stores sell reburfished game systems. years ago my son bought himself a used playstation when a new system was introduced to the market. He got it at GameStop and it appeared new to me, was in a manufacturer's box and even had guarantee.
My son got a DS when they first came out about 6 yrs ago. Then the ds lite came out - the screen were backlit - and my daughter got that about 3 years ago. Then the dsi - with a camera and wifi came out - we got that for my son a 2 years ago as his original DS was broken. Now both of my kids play with their ipod touch screen - since it holds music, movies, and is wifi enabled as well and rarely take out their DSlite and DSI... It's all obsolete within a year or two anyway! Are we nuts?
If I knew then, what I know now - I would have bought used stuff for them when they were little - they were much more rough on things when they were younger. Nothing like seeing your child drop an expensive digital toy...(about the same when you have to dig through garbage to find their retainer!!!)