For this to be fair, let's assume I own neither...
I would pick the iPhone. It's something I can take with me easily everywhere. iPad, no so much. iPad (tablets in general) also aren't the most convenient devices in the world when you are out and about.
I own the iPad 2 and iPhone 4s, but if I owned neither I would choose the iPad without hesitation. The iPad is in a league of its own while their are plenty of other phones that are as good, if not better, than the iPhone. Just my thoughts.
Definitely the iPhone. I can take it everywhere and thus I can have it do everything I need it to. It runs my work schedule, it entertains me with movies, tv shows, music, and games, it runs my social schedule with beautiful and easy to use apps that I actually enjoy using to communicate, i can use it to always have accurate info (scores, showtimes, news alerts) and it has so many great little uses that I love to always have in my pocket such as a calculator, flashlight, camera, GPS (WAZE), radio (PANDORA/SPOTIFY), etc.
No comparison in my opinion because of the portability.
I've got both. My PC has been out of commission for a few weeks and I've had to live with the iPad for Internet and let me tell you, I've grown to hate the damn thing. At best, it's a half-assed imitation of a proper laptop.
Large part of the web doesn't work on an iPad, the keyboard sucks when you know how to type, but are forced to hunt and peck and nothing even comes close to being able to use a proper, accurate pointing device like a mouse. I won't even go into how you can't play any decent games on the thing.