There are benefits to both options.
Single Order with Port:
Requesting the port as part of your order is very easy and saves you some steps but note that during the process due to the large number of activations / ports that are happening during the initial lanch time, there may be a delay between the activition of your new Verizon service and the porting of your old phone number. This means that you would be able to make calls from your new Verizon iPhone 5 as soon as the activation steps complete but calls made to your phone number will still be sent to your AT&T phone for 24-48 hours.
It happens every year due to the hugh number of port requests.
New Verizon Order with a delayed port:
This option will get you a new temp Verizon phone number, allow you to verify that Verizon service works well for you. Then you request the port of your AT&T phone number a few days after the big launch rush. If you don't like the Verizon service after testing it for a couple of days you can cancel the Verizon service and return the Verizon iPhone without impacting your existing service.
Dave