I carry both my iPhone and iPad with me at all times. My iPad is a wifi only model as I can always add tethering to my phone plan if the need arose. While I'm out and about and not in a hotspot, anything that needs internet is done on my iPhone. Anything else is done on my iPad. I don't feel I have any trouble switching between the two. I get The Daily on my iPad, which loads up automatically every morning as well as Wired, Engadget Distro, etc. And I have a lot of awesome games as well. I would say even in non-internet areas, I am still on my iPad a lot more than my iPhone.
I will say that when I'm at home or in some other wifi hotspot, I will ALWAYS grab my iPad before my iPhone. At home, my iPhone is reserved for non-iMessage texting, phone calls, and Home Sharing when we plug it into the iHome. (We also have an iPod Classic and an iPod Touch for that purpose).
So I guess my preferred device is usually the iPad, but the iPhone's retina display is so awesome, I don't mind being on the smaller screen. I still love the hell out of my iPhone 4.