T-Mobile question

Hello-

My wife and I were thinking about switching to T-Mobile to save some money on our cell plans (currently on AT&T). I was browsing around their website and discovered this whole "device payment" thing they are doing. Sorry for sounding really uninformed, but I couldn't figure anything out on their site. Is there no way to do a traditional "pay 200 bucks for the phone, and here is your plan"? We were both thinking about getting an iPhone 5s, and the 80$ family plan, but when you add 40$ a month that we would have to pay for the phones (in addition to the 150$ each up front), we would then be paying more than we are with AT&T.... I'm not sure how T-Mobile is saving anybody money this way, maybe its good for heavy data users?

Any insight would be appreciated; this is my virginal Verge post. Be kind!