HTC HD2, iPhone?

3G and 3Gs are pretty different machines, primarily for speed. If 3G to use it with the latest firmware and the latest software is simply unrealistic, 3Gs — while you still can.
Next. Battery. It is not. Including in the 3Gs.
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In General, I would recommend to stay on the HD2 or buy a 4-ku. There is a problem of the screen, battery and speed resolved (or are not as urgent as the 3G/3Gs).¹
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\r¹ After each comma insert "I think", "in my opinion" or "in my opinion" at random.

Hammer, buy a suitable piece of metal on Android.
As I'm not poked in Japony — even after govnokod for 5.5 K rubles they seem to me useless bricks. Moreover, no ipad or laptop with makou.

If you change that too for Iphone 4 and IMHO the flea will be. This is from the point of view of common sense.
Well, if you really want\\really need the iPhone(for example for some softiny) then why not change.

change HD2 on Android, just change the firmware. There will be a new phone in fact :)

No, the 3G and 3GS, after all, will be worse than the HD2 in terms of performance. On the 3G to put the latest firmware is not recommended, as the latest iOS version is demanding on hardware and is aimed at 3GS and iPhone 4.
About 3GS you can still think and figure out, but I would, in your place, left the HD2, if its dimensions are not critical for you.