Sound is fine on the torch..just as good as the iphone 3G (i think similar if not the same as the 4 as well)
Camera is good, im very impressed with it, same with video..very impressed.
Facebook and twitter are both great on the Torch, easy to use and flows well. What swayed it for me was that the torch was free on my contract..however on the same contract for an iPhone 4 they wanted £250-300 up front. I know you said about going for the 3GS though.
Also, the new iPhone is likely to be announced on the 6th of 7th June, it will be on sale in september (i think, someone correct me if im wrong). So the prices of the iPhone are likely to come down after that if you do decide you want an iPhone.