Galaxy S is the 'next generation' of Android phone and would need to be compared to the HTC Desire HD rather than the HTC Desire - the screen is bigger but also higher resolution (so in the same 'league' as the Apple Iphone4).
I've got a Desire and it's my first proper smartphone (I'm discounting the N95 I had as it was a glorified phone for me rather than something I use for more than just phoning). The concept of a smartphone is brilliant - loads of apps that can be installed on-the-fly (well once you have found them!) and use them...it makes the device a lot more than just something to make/receive calls with and is a great step forward.
To be honest, I'd suggest any smartphone is the same as any other in terms of brand i.e. Android is Android is Android and is very similar to Apple's Iphone - as a smartphone concept.
I believe Android could multitask from the word go and the later versions are likely to improve the multitasking/power management/task management and utilise more of the features that the newer smartphones come with i.e. I think there are some extra sensors in the HTC Desire so there are some apps that make use of these - the same app is available for the Desire but it isn't as 'functional'.
Android is a very good platform - it's generally free (or far cheaper than Apple) - it's flexible and expandable (same as Apple) BUT the big difference between Android and Apple is that Apple ties you completely into the Apple way of doing things, however, that does mean that any app you buy from Apple will have been tested and passed - Android isn't the same in this respect - they do not central testing to ensure an app does what it is meant to do, however it does allow you to install from multiple places.
Android phones tend to have more than 1 button to access content...
Almost every smartphone I have experienced (personally and from friends, etc.) all have issues with poor battery life - this is the bit I really don't understand - they seem to be pushing these devices as the next best thing and can do so much and yet they put some real poor performing batteries in them so they tend to only last a day - switch everything off and you may get a day and night out them but it seems such a stupid thing to not develop/improve - really smartphones these days should be going a minimum of 2 days between charges without skimping on stuff - but it just isn't happening.
If you are going Android you are unlikely to go wrong with any model - just choose one that comes closest to what you are after...
They are very good devices.