HD is the work horse of HTC. It remains up to date. Great camera. Great everything...
The Desire S has a smaller screen. Comparing phones you should look at the alternative, the Incredible S. Or even better, the Sensation. This one is tops.
In my opinion, The HD is still the greatest if your choices are limited to the Desire S and the HD.
Delfim M Cardoso -"The system of nature, of which man is a part, tends to be self-balancing, self-adjusting, self-cleansing. Not so with technology. ~E.F. Schumacher"