That depends on a couple things:
1) Will you be buying it discounted with a 2-year contract? If so, you might regret not being able to get a discount for 2 years if you get the 4S now. (on the same note, if you do not go with a contract, do you typically use the same phone for at least a year or two? If so, you might regret not waiting a few months considering new features it will have).
2) Will your current phone last a few more months? ...if it is falling apart or if it is not working properly you may want to go ahead and get the 4S now. The iPhone 4S is nothing to sneeze at.
The next iPhone will likely be out this fall and most likely will have LTE ability, if you live where this is already available this might be a consideration. It is also rumored to have a metal back instead of glass, and I've heard of a lot of breakage on 4/4S glass backs. (I sometimes drop phones, this would not be good for me as I don't want a bulky case on it, like it to slip nicely into my pocket.)
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Another option, (GSM networks only), if you have a friend that has a 3GS they are not using and they don't care about selling it, maybe you can rent it from them for a few months (or maybe they'll be nice and sell it to you for cheap). I have such a friend

and, I have to say, it runs iOS 5 pretty nicely, though doesn't have all the features, it will get me through. I, personally, can't wait for redesigned with metal back, and some of the other features it's rumored to have.