iPhone resale values

I have an iPhone4 taken out on an 18month contract when it first launched.

My wife has a six month old 4S. I have been very impressed with it and have been tempted to upgrade but decided to wait for the 6 /4GS/ whatever its called.

Today I had a call from my carrier offering me a free 4S on my current (£36/mo) contract if I commit to 24 month extension. They will also give me £234 cash for my iPhone 4.

Assuming the new iPhone retails for the same £499 off contract, I only need to sell the 4S for £265 to be no worse off.

Its a no brainer, init?