Depends on what you want.
IPhone is great if you just want to download apps that other people make.
If you like to tinker, and like to customize your phone, software wise, HTC HD2 is easier to customize and build your own apps, set it up to your wants. And if you need to change the battery, you can with HTC with little hassle.
I am a big fan of HTC and they are all I buy.
