I was in the exact same situation...
I decided to buy the 16G 3G iPad (instead of the 64G) so I can buy the iPhone 4. After I got my iPad, I don't have the urge to upgrade to iP4 any more. My wife got it so I can play with it whenever I want most of the time but I don't feel the need for it any more. Don't get me wrong, it's something nice to have. I now have switched over to pre-paid and I carry my iPad all over the place now. Granted it's larger than iPhone and can't fit in a pocket but it fits alright in my little bag. And I just can't go back to surf the web or read/reply emails from the iphone any more. But that's just me.