Galaxy S all the way.
People complain about lag on it ( but even with a ton of apps running, I haven't seen it - SA firmware maybe? ), but in the end, the screen.. You just have to look at it once to understand just how special it is to work with. The color is clear and crisp, no matter the lighting.
As for apps and stuff, it supports *everything*, so whatever you're looking for is easy enough to find on the market, and paid apps are coming to SA soon, says google.
In the end, my brother got the iPhone 4, and even he is now leaning toward the android camp after seeing that there's literally no reason to not have one.
The "gorilla glass" protecting the screen is nigh on indestructible ( a friend figured he'd be clever and attack it with a headphone jack, didn't even leave a mark, and he is also an iPhone 4 owner, so he wanted to see it fail.
Oh, and don't forget the weight. Because of the Super AMOLED display, the phone is thinner and lighter ( by about 40% ) than the iPhone, giving it less bulk, and more battery power.
I am very biased, but I weighed up my options ( and very nearly bought an iPhone, actually ), but settled on the Galaxy S.
EDIT:
The Desire HD is a great phone, but the Galaxy has a more advanced processor, better screen ( though slightly smaller ) and more onboard storage, I wouldn't wait for it if you can find stock of the Galaxy S.