You can choose outright as the way you want to pay for the device. You pay an upfront fee and the device is yours right then and there. The only thing you will have to pay for is your plan after purchasing your device outright.
When purchasing this device, are you going to stick with your current carrier or are you wishing to switch carriers?
If you are going to stick with your current carrier (AT&T), I would visit one of their nearest stores, their website or call them on their customer service line to purchase this device.
Click here to Compare AT&T iPhone 6s 16 GB Plans
Now, if you are wishing to switch carriers, make sure your current device is unlocked so you can trade it in for eligible credit towards a new device! This could be very beneficial since you are paying for the device outright!
Click here to Compare All iPhone 6s 16 GB Plans
Here are some carriers from that search results page that might be beneficial to you! From these results, you will be able to compare data inclusions, minutes/messages and prices to choose your best options.
When it comes to transferring your number to a new phone/carrier, that is the easy part. Simply let your new carrier take care of the porting process. When you activate a new device it will automatically happen. DO NOT DISCONNECT YOUR SERVICE BEFORE SWITCHING!
You can click here to see the WhistleOut Guide on Switching Your Number