I'm told, Verizon will open it's door at 8 AM on the 21st. That is here in Florida. Unknown if where you live it will be different. Have to get close to sale date and maybe it will be posted some where or you can call them and ask
Advantages for me, is that Verizon store is close and Apple is not. I can even get AppleCare + after receiving the phone, activating it and calling AppleCare + rep.
Disadvantages is that Apple I believe has a 30 day window to return it, and Verizon is 14 days.