Whether it jailbroken or not, the 3GS is a much better device. If you can figure out any way to swing it I suggest you get the 3GS. Better camera, twice as fast, etc.
It is painful to use my wife's 3G.
Of course, if you are buying new, the 3GS will most likely have the new bootrom and you won't be able to jailbreak it (conveniently anyway). At least until someone figures out a way to do a tether free jailbreak.
I won't upgrade my wife's in January when it is eligible if they haven't figured out a way to do a full fledged JB of the new 3GS'.