I just upgraded from a 0 SS to a S3 SS and I have to say it's a massive upgrade. I'm not on a carrier that supports the watch (only one in the UK) but I didn't really buy it for that. The thing is snappy and a pleasure to use though, the altimeter adds an extra bit of information to the exercises and the speaking Siri is also cool.. those are the major observations so far.
So, I decided that as I had a SS watch that I had to get another SS as I felt more comfortable with that. In terms of turning the old watch into cash I found it better to find a local trade in company that bought face only and that gave me the additional advantage of keeping the black leather classic bracelet which I could optimictically think offset the cost of a new watch by £130 because that's how much they used to cost on their own... better than fussing around eBaying IMHO.
My final thoughts as I was driving to the Apple store were that I had the watch for 877 days so anything else was a bonus but it would end up in a drawer when the software demanded too much of the hardware or the battery started to show signs of aging.. time to get rid of the old one was a big decider.