I have an iphone 3gs and just got an htc desire hd this week.
If I had to sum it quick the biggest advantage to the htc is the screen size. I have to be honest here, I am getting closer to 40

and my eyesight is suffering.
It is incredible what an extra 1 inch does to the screen.
Therefore
iphone: better apps, closed system, still no flash player!
desire hd: great screen, plus android is cool :-)
IMO go for the htc if you use it more as a smart phone and iphone if you use it more as a phone ! Does this make sense?
ps. keep in mind that the Desire HD battery is pitiful small and might but just might last a day with 3g data and wifi enabled.
rgds