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I say stick with 3GS if it makes you happy.

My younger brother has asked me how much I love the iPhone 4. He had the original since Nov 2007 and then moved on with a pair of iPhone 3G's. He needed to replace one after losing his first one and that newer one is now his current phone. I tell him the features from the 4, but it doesn't seem to interest him. He doesn't care for much about the camera or screen. He actually doesn't mind downgrading to a cheap Motorola RAZR V3i because all he really cares for is some music and texting his friends. He isn't an app whore and wants a lighter compact phone he can easily slip into his pocket than his current 3G. He still plans to keep his iPhone to use as an iPod touch and as backup phone. Sometimes people don't need all that in a phone and just want to go back to the basics again. Perhaps after three years, he is also getting tired of iPhone and seeing it everywhere now too.

But if I had to choose between a 3GS vs 4, I would pick the 4 if I never used them both. But since you already used the 3GS and it makes you happy, then some of the upgraded features may not be all that worth it for you. Might as well wait for the 5. It all depends on how much value you have with those new features and if it is that important to you.
 
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