Sure, you're buying a portable device for gaming. But sometimes, it's nice to just have one gadget to perform a multitude of functions. While the iPad does offer a great gaming experience, it's built for a whole lot more. Productivity applications, photography editing--as corny as it sounds, it's fair to say in most cases, "there's an app for that."
The PlayStation Vita also does a spectacular job of displaying media, thanks in no small part to its 16:9 widescreen display. Movies and other entertainment are loaded onto to Sony's proprietary media cards, and are easy to watch. The Nintendo 3DS offers a few functions, like Netflix, but the only feature to stand-out is the 3D camera. Granted, the pictures themselves are a bit low-resolution, lessening the importance of that unique element.