S4 on AT&T vs. T-mo

Hello,

It looks like the S4 will be my next phone, so I've done some calculations with how much I would pay if I stayed an AT&T subscriber vs. the new "Uncarrier" T-mobile plan.

AT&T:

450 minutes @ $40/month

pay per text @ $0.20 per text/$0.30 per pic/video (I barely use text messaging)

3GB data @ $30/month

$250 device cost

= $250 + ($70 x 24 months) = $1930 + maybe $10 for 50 text messages.

Versus.

T-mobile

Unlimited Talk & Text & 2.5GB data @ $60/month ( I never go above 2GB a month)

$99 device cost + $20/month installment or just $580 upfront, but I don't have the luxury of paying that much upfront.

$99 + ($80 x 24 months) = $2019.

T-mobile manages to be more expensive (but not by a lot).

Seems that only advantage to being a T-mo user will be having the option of leaving whenever I choose (assuming i've paid for the device cost in full).

So other than being locked down into a 2-year contract with AT&T, I don't see a reason to switch to T-mobile.

Please feel free to point out any mistakes I may have made in my calculations or even persuade me to go the T-mobile route :) (I live in DC/Maryland Area, if that matters when taking coverage into account).