I have tried the comparison - the difference doesnt work out to be that much difference in my opinion. Obviously having a stronger processor is a plus - and $100 is not that much a difference. I choose the Legend for its looks, and small size.
I always have it in my pocket and the small size is a plus. I dont see a noticeable difference.
Than again, if you plan on doing some hardcore multitasking, with many many apps - etc. The other aspects of the Desire might be useful. I believe it has larger RAM etc. And with its larger screen, if you will be watching a lot of vids/movies, than it might be the better choice.
I am waiting to see what else will be available in the next 6mths - 1yr - Samsung Galaxy S just came out with the 1ghz processor - whats next?
I think HTC will come out with a higher end phone over the Desire, as i think this is their top tier phone at the moment.