I use an iPhone 4. What do I like about it? Camera, pictures, browsing, music player, text messaging, email (and can monitor all my accounts), availability of Apps, and CROSS PLATFORM usability - in other words all my music, pictures, apps, email, videos can be viewed on my iPad - at no extra cost. My calendar updates back and forth between devices, seamlessly.
I waited a long time for Apple to get it right and they did. If you want a bigger screen, get a tablet. If you want the BEST tablet and the BEST phone go with iPad/iPhone. It just works!
PS - You don't need 3G for the tablet either, my iPhone 4 allows me to tether the iPad when I need to access the internet on the tablet and there is no WiFi available. So no extra contract cost for the iPad either. 'Nuff said.