Personally, I believe that the only real difference between the iPhone and iPad is the UI and UX . Most of the code is the same for both applications. (In fact, you can even reuse a lot of code if you are making a universal application!)
The main difference is the screen size. The iPad is huge compared to the iPhone, and this difference gives you more room to work.
Another thing to note is that the iPad is usually done in landscape orientation, while the iPhone is in portrait mode. However, this is not an absolute rule, it is the way I see it.
The graphics are not too different between devices, except that due to the large screen size, you can work a bit more with iPad images and then with iPhone images. (Even with the retina display, too subtle details on the iPhone are barely noticeable.)
Another thing I wanted to mention is the flow of devices. Again, due to the size of the iPad screen, you can place a lot more content on the screen, and so you have to click on new views more often. I worked on applications where we had 3 UIViewController for the iPhone version, and only 1 for the iPad version.
In short, it really depends on your type of application. If you have a lot of content and want to display it right away, go to your iPad. If you want an optimized minimal approach to your application, go for the iPhone.
Again, this is all my personal opinion. Hope this helps!
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