Dimensions
The iPhone is slimmer than the HTC Desire, making it easier to fit into pockets, even though it is slightly heavier.
Display
iPhone:3.5", 960 x 640, 326dpi, TFT-LCD
HTC Desire:3.7", 480 x 800, 240dpi, AMOLED
Even though the HTC Desire is slightly larger, the iPhone has a higher resolution and pixel density. While the AMOLED might use less power and have arguably better video performance, it's not as good in sunlight.
Battery
iPhone: 47 hours 3G calling, 300 hours standby
HTC Desire:6.5 hours 3G calling, 360 hours standby
Both batteries are the size they need to be to run the components inside them.Apple are claiming a bigger battery over the 3GS and improvements with the A4 processor. That means that while it offers a longer call time (half an hour), the HTC Desire wins out on standby.
Imaging
iPhone: 45MP, flash, AF, 720p video capture
HTC Desire:5MP, AF, flash, 800 x 480 video capture
The iPhone's got itself a LED flash and better sensor and lens than the older 3GS, but what makes it really good is it can capture video in 720p resolution. There also is iMovie for iPhone, (although you'll have to pay extra). Even so, the iPhone probably wins this one.
Storage
HTC Desire: 32GB microSD + 512MB
iPhone: 32GB
The HTC Desire wins this one by a small margin, mainly because while the main 32GB is the same, the HTC Desire technically has more memory and you get the ability to swap out micro SD cards, where you can't with the iPhone.
Price
The iPhone is definitely more expensive, so the HTC wins this one.
Conclusion
In my opinion, the iPhone 4 is a better phone than the HTC Desire. It's better shaped, has a better screen, you can fit more on it and, arguably, it looks better too. That doesn't mean that the HTC Desire is some kind of inferior mobile because it's not. It's just different. Many people would prefer the iPhone 4 as it might suit their needs better, but others would prefer the cheaper price and the originality of the HTC Desire.What you have to ask yourself is which is the one for
you.