The whole - "T-Mobile is cheaper than other carriers in the US" - myth

Can someone explain exactly how T-Mobile is supposed to be cheaper? I am currently on a two person 4GB AT&T Mobile Share Plan where we pay $150 per month [$70(4GB plan)+ $40*2(for 2 smartphones)]. If I we switch to the similar T-Mobile 2GB simple choice plan our month charge is...........$150! [$80(simple choice plan)+$20(2GB plans for each phone)+($25*2 for both smartphones)].

So exactly is the T-Mobile plan supposed to be cheaper? Granted it is more transparent and the monthly bill will go down by $50 after the phones are paid off, but who really uses a phone beyond 2 years these days? I think the whole "T-Mobile offers the cheapest plans in the US" mantra has been over-hyped. They are not really cheaper by any means for the first 24 months. Anybody else feel the same way or have your own examples?