What's the best way to upgrade to the iphone 6s?

Right now, i have an at&t iphne 5s, and i'm upgrade eligible on October 4th. But i wan to get the iphone 6s 64gb, and i was wondering what would be the best way to buy? The att next plan, or the apple upgrade plan directly through apple?

One thing about doing the apple plan, you are never really locked to any carrier so its separate. Not that people change carriers often anyways, but its still a little more flexible.