You'll save a lot of time doing it in Photoshop/Illustrator. If it takes other screens and API requests to get to the UI you're building, it'll take a while before you can see what your code does. Using images, especially stretchable ones, is also lighter on memory.

Also, I don't know what you mean by "scaling." You should be able to zoom images with UIView animations, and should have different sizes of the images depending on whether you're targeting an iPhone/iPad.