For me, I like Android and have no experience with Win phone 7. I currently own an iPhone 4 and the lack of flash is just irritating, while yes there is Skyfire (a browser that lets you watch flash videos that they transcode, and is currently being released in batches due to server overload as everyone with an iPhone wants it), it's not a true flash experience it just let's you watch some flash videos if it recognises them. It's very annoying when browsing the net.
Apart from that I must say that the iPhone 4 is great overall, wonderful screen and a nice design etc nothing you haven't heard before and/or experienced with your own iPhone. I upgraded from my 3G to the 4 and well I was quite underwhelmed lol but I knew I would be
. Phone is faster, screen is great and with a camera app like ProHDR you can take pretty good photo's also stability with apps like Skype is better as it's better able to cope with it, but that's from 3G to 4 the difference will be less pronounced from 3GS to 4. I have no reception issues, but battery life is markedly poor in comparison to the 3G and 3GS at least for everyone I know with one. That's another thing, if you like to have a good phone that isn't what everyone has then another iPhone isn't for you as it's like a Nokia 3310 now.
Problems, well the UI is boring and I like empty backgrounds at times lol (hence having no icons on my desktop just a rainmeter setup and a nice wallpaper), it sort of ruins an upgrade experience when the OS is so similar in an odd way. I say odd because normally continuity is great but with the iPhone I find it to be almost as if they've lost some innovation, at least there are folders etc now but again you'll have that on your 3GS. If I could dual boot Android properly I would but then again if I could I'm not wanting to risk a contract phone.
Both HTC and Samsung would have my vote, unless you know that you like the iPhone but tbh if you knew it you wouldn't ask about your options
so maybe you're bored of them! Don't get me wrong though it is a wonderful phone
. If you wanted a larger phone/tablet you could get the Samsung Galaxy Tab with an HTC Desire one contract for £350, sell the HTC for £350-400 and stick the contract sim into the Tab and use it for browsing, voice etc when the gingerbread update comes the Tab should be looking really good for those wanting something a bit bigger and don't want a tablet + a phone. That's why I'm not looking at a tablet for now, although selling the iPhone is tempting the Galaxy Tab is rather large to carry! Good luck with your purchasing decision, there's drawbacks to all the phones I'm afraid
but the HTC's are nice and so are some of the Samsungs, also maybe look at the Dell Streak it's surprisingly ok.