Did you fail math? Tmobile /w 2.5 gb including monthly payments towards phone is 80 a month and no activation and 149 down towards a s4. This goes down to 60 a month after its paid off.
Verizon is nearly 100 as is att and neither drop after 2 years. Plus they cost more down and have 30+ activation fees.
I realize I'm in a better position because I have favorable grandfathered rates. Let's look at a new customer to either AT&T or T-Mobile. Let's take the 16 GB iPhone 5 as an example.
Let's use the 2.5 GB you even quoted.
T-Mobile for 2 years using a 2.5 GB plan unlimited talk and text.
$99 (iPhone 5 price up front) + $400 ($20 a month, 24 months) + $1,440 ($60 monthly plan, 24 months)
(Your monthly bill would be $80.00 a month without taxes and surcharges.)
Total: $1,939.00.
AT&T for a new customer. Same phone.
$199 (iPhone 5) + $936 ($39.99 a month for 450 minutes, free nights and weekends) + $720 ($30 a month for 3 GB) + $480 (Unlimited texts) +$36 (Activation Fee)
(Your monthly bill would be $89.99 a month without tax and surcharges.)
Total: $2,371.00
Difference in price: $432 over 2 years. Or $218 a year.
For $218 a year more you get an established LTE network. I'm not saying the T-Mobile plan is a mistake, it's just not a huge savings per year in such a common scenario.