AT&T gives me an upgrade after 18 months
AT&T doesn't give you anything you don't pay for. Here's a complicated (and hopefully accurate comparison of costs with the new family/shared T-Mobile plans). I wrote this for people with little tech experience or interest so hopefully it's not too inane.
T-Mobile: Unlimited talk/text + 500 MB / month of high speed data for $50 for an individual plan.
However, under a family plan, you can have 2 lines for $80 / month, 3 for $90, 4 for $100, and 5 for $110.
This means that with 4 lines on a plan, cost is $25 per month + taxes/fees (probably about $3.75 per line). If you need more than 500 MB of data it's $10 / line extra for 2 GB more (added per line so it can be added to just 1 or all of the lines separately). Frankly, most people will not use more data than this especially if around WiFi much of the time (if you do use more data, pay extra for more).
Options - Buy a used phone compatible with T-Mobile (e.g., iPhone). One of the best options (non-iPhone) is the Google Nexus 4 for $300 or $350.
For a higher upfront cost, with 4 lines on a plan, you are only spending $300 a year on the cell plan compared to the $750 you would spend on a comparable Verizon plan (shared family plan with 4 lines) - plus you are locked into a contract with Verizon or AT&T. With those providers you can get a free phone (although most of the phones worth getting cost at least $100) but even so, on T-Mobile (4 line plan) with a $300 phone + $300 a year, you've spent $600 compared to $750 (free phone) - $950+ (e.g., purchasing iPhone 5).
Over a 2 year plan, 1 line (on a 4 line family plan) is $1500 (assuming you opted for a free phone) or $1700 with an iPhone 5. Compare this to the 2 year cost on T-Mobile of $900. So you've "saved" $600 - $800 over the 2 year period (plus you get the freedom to cancel whenever you want to with T-Mobile should you need to cancel for some reason). This money could be saved or even used to upgrade your phone in 2 years (and have money to spare).
Granted, Verizon or AT&T (comparable 4 line plan cost is $57.50 / month) have better coverage than T-Mobile but is it worth an extra $600 over 2 years?
As an addendum, many corporations or universities offer discounts for employees (at least for Sprint, Verizon, and AT&T). So here are the numbers with the discounts on both AT&T and Verizon:
AT&T with typical 15% corporate (university/hospital) discount = $49 / month
Verizon with typical 15% corporate discount = $53 / month
You still save $25 per month, which covers the cost of a good phone in as little as 4 months (when getting a $200 phone like the iPhone with an AT&T or Verizon plan) or as long as 12 months (with a free phone that might be worth "free"). But this means that you are still locked in for a 2nd year and so after that 2nd year you are "down" $300 (again this is factoring in corporate discounts). Maybe a particular phone is worth that to you. For example, if you want the iPhone 5 and can get a corporate discount, it can be better to sign up for a plan from AT&T or Verizon assuming you value the speed and extent of AT&T or Verizon networks over T-Mobile (depends on location) - here's the math. On a 4 line shared plan with a 15% corporate discount, AT&T + iPhone 5 costs $1376 + taxes/fees {typically 15%} over 24 months. To do the same on T-Mobile, you spend $650 for the unlocked iPhone 5 + $600 for 24 months of service, for a total of $1250. So, if AT&T's network coverage and speed is worth an extra $5.25 / month, it can be as good or better to go with AT&T. But what this means is you can buy an iPhone 5 from Apple outright and still come out ahead on T-Mobile.
Things are even better with T-Mobile if you have 5 lines on a family/shared plan ($22 / month per line).