The Sensation is a boat load faster than the Desire S, but both are good phones.
http://www.knowyourmobile.com/compar..._desire_s.html
"Put directly: if you want the most visually pleasing, powerful Android smartphone around, get an HTC Sensation. (If you want pure power, get a Samsung Galaxy S2). But if you want a device that offers a nice balance of features, while maintaining great value for money, give the HTC Desire S a look, because it may not have won this battle, but it certainly has a lot to offer."