If you wait,then next year you will have to pay the early upgrade price again, instead of the new contract price that everyone who bought 3G's on release date are getting the new 3GS's, get it now for the early upgrade price and next year when theres the new iphone of course, upgrade and get a new contract price....
as far as the changes in 3G to 3GS, to tell you the truth, I did upgrade, I dont use Voice memos (RARELY), Compass(sometimes for literally 2 secs lol), a lil more on the maps but still not that much, Battrey % is cool, but jailbreak can do that, Video camera tested once and thats it, auto focus helps but still rarely use it, Battery Life is WAYYYY better, phone is much much faster, when you click settings you dont even need to wait....Internet pages load faster, apps as well,
bottom line is it revolutionary....no, is it worth it compared to 3G, i Say yes