There are so many different variables when making that call. You have to evaluate all those variables from what you want out of life.
Personally I do not want to be tied into a contract. Just my trigger. I had a 4, so surfed the web until I was able to get a 4S for the $ I wanted to pay. The 5 is just too much money for me to pay and not be on a contract. Again, these are my variables which influence my decisions. Works for me, might not work for anyone else.
Are you afraid of the break-ability of the 4S compared to the 5?
Do you often wish you had faster data speeds?
Do you want slightly better battery life? (This might be an individual situation, but my 5 gets better life than my 4S ever got)
Do you want a more future-proof device?
Do you have the money to do it?
Do you ever find applications/webpages load too slow, or see a lot of lag?
Those are some of the improvements of the 5 over the 4S, so if you answered yes a lot, you may find the 5 to be worth it. If you are fine with the performance, speed, battery life, etc of the 4S, it probably won't be much of an upgrade.