I won't comment on the issues of plan prices, but I would like to make an observation about the phones themselves. A 4S is essentially a refined 4, with the 4's design flaws addressed. For example, I and many others have had a horrible time with the Home button going faulty on the 4. I had the phone replaced twice under warranty for the same issue--the home button quit working--and my second replacement eventually had it's Home button fail also....only this time it wasn't under warranty. Apple admitted a design flaw, which I assume they addressed for the design of the 4S. If the home button on 4 goes out, you're looking at either limping along with iOS 5's assistive touch menu (annoying) or paying around half the cost of a new subsidized phone to get it fixed, with no guarantee that the problem won't recur.
Then there's the so-called death grip issue. It was really only a problem in low-signal areas; the thing, I and lot of other people a lived low-signal area, so it did become a problem. The antenna design of the 4S specifically addresses this.
And to top it all off, the camera on the 4 had HORRID indoor white balancing; pics taken under tungsten lights were so badly yellow that it couldn't be corrected even in professional post-procesing software. The 4S is not anywhere near perfect in indoor lighting conditions (even professional cameras have difficulty with auto white balance indoors), but it is a noticeable improvement and knocks the yellowing down enough to where the white balance of the resulting photos are reasonably correctable in post-processing.
Oh, and the 4S has a much improved speakerphone and noise cancellation.
So if you're going to be paying out even roughly equivalent amounts of money, and all other things being equal, I'd go for the 4S.
Thanks so much.