Why would one place one over the other? To me, they are both computers...the phone can talk, text, and take pics/videos. It can be with me all the time, in the pocket & car. The iPad, not so much, but I use it for more work related things, so it gets used probably more than the phone, but it different ways. I take pics on the phone and view them on the iPad. Works great. I really enjoy photo stream (until it pissed me off) ability to let me snap on the phone and view on the iPad, almost instantly. If I am in a place where I would use either one, I will pick the iPad because of the larger screen. For some things the iPad is the go-to device, for other things, the iPhone is. The integration between them is useful.
Frankly, I would not be without either of them if I have the choice. Apple should sell them as a package with a slight discount if you buy them this way. Of course, they have no motivation to do so, but I think they might attract a certain segment of buyers who value the integration.