I tried using my brother's iPhone 4 coming from my Samsung GSII and it was horrible browsing the web. I'm sorry but it felt so restrictive relative to my experience with the GSII.
I also played a HD youtube clip on both and the iPhone looks so washed out in comparison.
The iPhone is a beautiful device, but I couldn't live with it's smaller screen personally. The GSII isn't bad looking either IMO. It has a much smaller bezel so that the screen to size ratio is pretty impressive.
Not to mention that the GSII is much cheaper than the iPhone.
It depends on the person really which they like; one person may say that IOS is too restrictive, while another may prefer its "simplicity" and find the more customizable Andriod OS too complicated.