HTC Desire was the favourite among the HTC family and it comes with siblings with its own function that suits different people with its advantages and disadvantages. The most recent release of the HTC Desire sibling, HTC Desire S was born to give better improvement in the HTC Desire line.
I pretty sure there are a lot of questions pertaining how much HTC Desire S has improved from its siblings. I'm only here to go through the comparison briefly among HTC Desire S and the other Desire(s).
I may not be a very good reviewer that can do comparison, but I hope this could help a little. Or if not, I'm sorry and check out more reviews and comparisons out there.
HTC Desire S vs HTC Desire
Similarities: Weighing around 130g, the screen size of 3.7 inches with resolution of 800X480. Running on 1GHz snapdragon Qualcomm processor. Both have HTC Sense UI, virtual keyboard and 5MP color camera.
Differences: HTC Desire is running on Android 2.2 (Froyo) while HTC Desire S is on Android 2.3 (Gingerbread). HTC Desire has no secondary camera, which is available in HTC Desire S a VGA secondary front camera. HTC Desire comes with 512MB internal storage + 4GB microSD while HTC Desire S comes with 1.1GB internal storage + up to 32GB external storage.
HTC Desire S vs HTC Desire Z
Similarities: Both has a 5MP camera with singel LED flash, video 720p HD and HTC Sense UI. Standing together with the same screen size of 3.7 inches.
Differences: HTC Desire Z is running on Android 2.2 (Froyo) platform with 800 MHz Qualcomm processor, 512MB RAM and 1.5GB of internal storage while HTC Desire S has a platform of Android 2.3 (Gingerbread) with 1GHz snapdragon processor, 768MB of RAM and 1.1GB internal storage. HTC Desire Z has no secondary camera. It also comes with a physical keyboard. Speaking of the the design, HTC Desire Z is thicker.
HTC Desire S vs HTC Desire HD
Similarities: Having the same RAM of 768MB, supports microSD & microSDHC of 8GB and running on 1GHz Qualcomm snapdragon processor. Both with HTC Sense UI and a virtual keyboard.
Differences: HTC Desire HD running on Android 2.2 (Froyo) platform. Its display is a 4.3 inches (BIGGER) touchscreen S-LCD (Sony's own technology) while HTC Desire S has a 3.7 inches (SMALLER) 480X800 resolution touchscreen with multitouch and proximity sensor for auto rotate. HTC Desire HD has bigger internal storage of 1.5GB vs HTC Desire S internal storage of 1.1 GB. HTC Desire HD has a camera of 8MP with dual flash. It is seen that HTC Desire HD has a shorter battery lifespan compared to HTC Desire S especially using the Apps.
That's all I have for now