Wait wha-? I don't mean this in an insulting way but those saying iOS6 (or 5) will run fine on the 3GS and the iPad 1's performance on iOS5 is great obviously has had not experienced any of the newer devices in both the iOS and Android camps.
My iPad 1 on iOS5 vs 4: Constant page refreshes between tabs, lots of browser crashes due to memory, having to constantly use the task switcher to kill off apps or large pages will not load completely, HTML5 videos causing the browser to crash, etc. Switching to the new iPad with its 512MB (or was it 1GB) RAM was a MASSIVE difference.
My 3GS on iOS5 vs 4: Lag when typing on the keyboard. Very slow responses from apps like Downcast. Lag when navigating menus unless I kill off apps manually. Extremely poor performance with the new Podcast app though I think this is more of an application issue than 3GS performance. Other apps responding slowly when the music player is playing.
My 2010 MBA (not really iOS but...): Lost its instant on capability after switching to OSX Lion. Well, from near 0s to ~5-7s startup :\
Sorry but I do not see how upgrading to iOS6 will help in any way and if there is one thing that I've learned from owning Apple products, it is never to upgrade the OS too much. Apple after all is a hardware company and it is in their best interest to make customers upgrade hence the planned obsolescence with their OS updates.
As for me, I am just waiting for the next best iPhone to come out in October or if it is not compelling enough, maybe the GNex with Jelly Bean. Now THAT is a sweet OS with Google Now and Maps.