I bought myself the Desire and I really love it. I haven't held the Galaxy S in my hands yet, but it seems to be a very nice phone as well.
From what I've read so far, the Galaxy S has slightly better hardware, but the Desire has better software. Many people really seem to love HTC's Sense User Interface.
If I were again facing that decision, it would be tough for me. Looking at the hardware side alone I would definitely pick the Galaxy S because it seems a bit better. But apart from the hardware, the Desire seems to deliver a better experience so far. If that would never change, I would probably still take the Desire, but thing is: The big advantage the Desire has is said to be its great user interface. But that's something the Galaxy S might be getting as well and soon, as I've read that Google is concentrating on improving Android's user experience for the upcoming version 2.3 of Android. But nobody knows when 2.3 will be released and which improvements there will actually be...
By the way, the specs you may read in some HTC desire vs Galaxy S comparisons may be slightly outdated. Since HTC released firmware 2.2, the desire is now capable of recording HD video (720p) as well. And firmware 2.2 made the whole faster as well.
