I am just offering my informed opinion. The Desire already has a 2.2 with Sense from the xda community, and has had 2.2 Vanilla for nearly 2 months. The Desire will have 2.2 official with Sense in the next 6-8 weeks, the Galaxy S won't have it from Samsung much before christmas. The Galaxy S will never see official gingerbread, the Desire might.
The fact the Desire is soo close in dna to the Nexus one means it is likely to see updates from the community for far far longer than the Galaxy S ever will.
The Galaxy S is a lovely phone, and its specwise better than the Desire, but for the reasons outlined above I would never buy one.