JoD doesn't have a monthly fee.
It looks like it doesn't! So the $15/mo fee used in the above math was wrong. T-Mobile's JoD really is the cheapest way to get the iPhone.
And there is no 18 month constraint (as I implied above) -- you can trade in for a new phone a year later (or more often). The 18 month is just the principal pay off period.
So T-Mobile's JoD is the cheapest and allows you to upgrade to a 7.
The only annoyance is you have to call or go into a store to sign up... I wonder when their phones will be open for orders.