It’s got to be the iPhone 4 all the way. It is leaps and bounds ahead of every other phone. I was looking at the contracts over your side and they are expensive. I paid £90 for the phone and then £45 a month which includes 1000 mins, 750mb download and unlimited wifi. I have heard 3 is rubbish with the network coverage but you have O2 over there and here is the page

They are out of stock but should have some soon. A friend in work has a HTC which is ok and my girlfriend had a new Sony X10 and sent it back because it was rubbish. The big difference in the phones is not the OS systems but I think the screens. The screen on the iPhone is unbelievable. It is very clearer and crisp and the touch sentiviative is perfect. You can type very fast and always hit the right button but on the Sony or HTC I was makin mistake often and that was the main reason my girlfriend sent the Sony back, the screen was crap.
The iPhone will cost you alot more but you are getting alot more phone for the money.
Another thing with getting and smart phone is you will always be on the bloody thing. I get told regalur to switch the phone off in bed and work.
They are great and recommend getting one but has to be a iPhone 4 not the 3GS.