If you're happy with it I'd stick with it. The next iPhone update that's gonna come next week is gonna bring a slew of long awaited stuff and make snappier on web browsing too. Notable new things: cut/copy/paste, voice commands, search on the whole iphone (a la spotlight on the mac). check the whole list here
That will allow it it maintain its edge vs. the Pre and the g1 from t-mobile.
Now if you want more speed from app and games, the true GPS , then the 3GS is your toy...
The Pre seems nice but you'll be stuck with Sprint and a phone that works only in the US... From the first reviews, it's nice but a little bulkier than the iPhone and the whole experience with it still show lots of rough edges.
I was not that impressed with the latest update.. I was hoping to see more, but in light of the age of my phone I will likely upgrade to a 3g on the cheap and wait for the next "REAL" iphone update.. this one was mostly software upgrades surrounding minor tweaks to their hardware..