Hmmm...that's a tough call.
I think you should keep your city. The city is a pretty iconic bag and has been around for years. The Stam is really great, but it is significantly heavier than the city. Plus, it's tough wearing on your shoulder (as the handles aren't that long...unless you get one from Fall 2005) - esp. with the chain. I owned the Stam and had some problems with it (quality-wise) and had to return it. A couple of people here have also had problems with the kisslock not functioning properly.
I don't have the Gucci, so I can't offer much about it...but I remember it being a HUGE deal because Jessica Simpson wore it on her show a lot. I just don't find that open top appealing - I prefer bags that close shut or zip.
All three bags are pretty popular and have been copied a lot....but either way, I think you should keep the city as it is relatively light and durable.
Let us know what you decide to do and good luck!!!