Nothing much if you got a better smartphone. Only thing is that its bigger and better for use and easy to use as well with a little bit change in specs.
Both are good, Ipad are ideal for businesses and not for taking pictures, video recording and playing games.
iPad is a device which is included in the Tablet category, which is differ with the smartphones and its features. If we go with new released iPad 3, then we can see with its features that this tablet can easily beat all other tablets with its Retina Display feature. If your questions would be like this: "What is difference of the Smartphone and iPhone?" then it were more points with their compatibility.
In my opinion it is the Retina Display. It is so clear...and I love iMessage. I use iMessage a lot...even for work.