I very rarely ever stream Air Video over 3G. I do it over WiFi, when I'm out, at a hotel, at home, whatever... You can NOT beat the screen size of the iPad. Comparing the two is still madness. I don't care how high res the iPhone screen is (well, I do, but that's for when I'm out and about) - you can't beat the size. And Air Video is amazing over 3G or WiFi. Your experience is not typical, that's for sure...
PLUS, I'm glad my iPhone battery actually lasts a bit longer now that I have an iPad. Plus playing games on the iPhone isn't in the same league as playing them on the iPad.
I mean really, there's a huge number of reasons why you should keep the iPad.