Any experience in differences between 3g and 3g"s" Iphones.
Yes. If you have used a 3GS you won't want to go back to the 3G model as the 3GS is much faster.
If you are buying new, try waiting till June when the new model comes out, it will have a new multi-tasking operating system (retro installable on the old models), a 5mp camera and flash, amongst other features.
If you are Jailbreaking, the latest 3GS's won't work (you need a PC with iTunes to boot them up if you restart them). This has been the case for a few months. If you are buying second hand, aim for an early model 3GS.