I have a 4th generation iPad, and I recently bought my first smart phone, an iPhone 6 Plus. The iPhone is a great device, I'm really glad to have it. But it's not as usable as the iPad. Mobile versions of web sites are usually less useful than desktop versions, and if I request the desktop version of a site on the iPhone, it's usually too small to read without a lot of panning and zooming. Reading things like books and magazines on the iPhone is also problematic.
The iPhone's only advantages over the iPad are (1) more portable, and (2) it's a phone. If I'm at home, I reach for the iPad for looking things up or sending emails. If I'm out, the iPhone is perfectly adequate, but not as pleasant for some tasks - but it's always with me. It's good to have both.