Siri is cool, but the novelty wore off after a single weekend.
I'm returning my 4S and going back to my 3GS. My 3GS still works great. For me, the 4/4S doesn't do anything over and above the 3GS that I care about.
siri - maybe when it's better
facetime - used it once, probably never use it again
front facing camera - meh
alert center - meh
64Gb? my 3GS 32Gb is only 1/2 full
Id put the sim in the 3GS and use it for a day first. You might realize the difference is greater than you thought. I turned my 3GS back on and couldnt believe how grainy it looked. And its so slow.