For the iPad vs iPhone debate, I would consider these to be two different categories of products. You will have to think at where people use each device and try to think how your application will fit into these patterns. So grabbing such device will help you get into the testing your application as a simple user.
P.S. Currently I have an iPhone 4 which is the one I am stuck for now. However I am very happy with it, despite the fact that it very old, for using a dozen of specific applications it works fine (i.e. Nanostudio). If I were to purchase a newer model I would go for the iPhone 7 or 8, and only for keeping up with the OS versioning, not so much for the latest hardware features etc.