iPhone 4S SIM Swap -AT&T

Hello...

I have an iPhone 4 through work and my wife has an iPhone 3G through another account. Both are AT&T. My wife is eligible for an upgrade so I used her account to order a new iPhone 4S this morning. I am going to take the iPhone 4S and use it has my phone (with my work SIM) and she is going to take my iPhone 4.

So, onto my question. When the iPhone 4S arrives on the 14th with the new micro SIM for my wife's phone number, how should I activate the phone? Should I activate it with her SIM installed and then swap SIMS or should I put my work SIM out of my iPhone 4 into the iPhone 4S before activation?

Thanks for your help....

-Marc