The market of tablets are now no way legging behind smartphones. Amazing tablets at relevant prices are now a common factor in every corner. Same goes in regards of iOS devices too. According to me the prime shade of difference between iphone and ipad lies in context of the User Experience and User Interface.

A large fraction of codes remains the same in case of both. The main difference lies in the screen size of both the devices. Compared to iphone, ipad resembles a much bigger size. This itself allows the developers to opt for a lot more work in ipads. Another notable difference between the two is that ipad is developed in landscape orientation whereas iphones is done on portrait orientation.
The graphics of both the devices kind of remains the same, accept that being bigger in size, a developer can do more detailed work in an ipad. On a different note, while working for these devices, a developer can opt for much lesser view controller compared to iphone. Since the ipad screen is far bigger than iphone, one can at a time view much more content than iphone.
The main prime note still lies in the kind of app that is developed. If you want more content to be shown at a time, the developer can opt for ipad. On the other hand, for minimal approach to the app or step by step approach, iphone is always the good option.