It depends. If you are going to stick with the iPhone, and never move to a different phone, I think the Upgrade program is a bit cheaper year over year, by like 40 bucks or something. The Upgrade program cost more monthly, but you have a $150 down payment with T-Mobile. The upgrade program also includes Apple care, while T-Mobile charges extra each month.
I would go the Upgrade route, but I couldn't place the order online, and the stores are just telling me to come launch day, hence why I ordered via T-Mobile. If I see no signs of movment come Thursday night, I might swing into the Apple store first thing in the morning.
Last year, ordering a 6s Plus for the upgrade program was easy and quick, this year it seems like no one knows what is going on.