I don't.
I prefer the device I'm using at the time - and I'm using it for the simplicity and ease of use. Generally, I use the iPad for browsing the web, reading news and RSS feeds and for being on this forum. I use the iPhone to play games (Draw Something and Words With Friends being the main ones right now), check twitter feeds and check Facebook statuses/updates. And, generally, I don't mix the activities (e.g. I don't check Facebook on the iPad or don't read RSS feeds on the phone).
But, while I am on this forum, if I have a question I want to research, I use the iPhone so I don't have to get out of the app on the iPad. I can do research and check facts with the iPhone without missing a beat on the iPad. That's just one example for when I'll use the iPhone while actively using the iPad - or vice versa.
As was said - the question assumes I prefer one over the other, but it's not that for me. I'm just glad I have both and they work great TOGETHER.
Marilyn