With ATT it important to distinguish you get 15 GB for $100 plus $15 per line (plus your leases but we won't discuss that part too much).
You could get four lines with 10gb each for less than you're paying with ATT (assuming you don't have some sort of discount). We have three lines but use the sin of the fourth in a mobile hotspot in the car for road trips (may as well use ALL the data we pay for, right?)
Leases on devices are also cheaper than att anywhere from $5-25+ per month depending on trade in and when you join the promo. This may or may not be applicable for you as I have no idea how you buy and how often you upgrade your devices. They've also got a program now where you can upgrade up to three times a calendar year for $0 down and no additional payments (slate is wiped clean) so long as it's a completely different device (i.e. Can't jump from a black 6s to a silver 6s).
Lastly, consider that music streaming doesn't cost against your cap and, starting in the 15th, neither will streaming video in just about any major apps outside of YouTube (they've steadily added services to music freedom so I imagine video service, maybe even YouTube, will follow).
You're not wrong in that, for two, you're savings aren't astronomical. In fact, if you have a discount on ATT, they may not be a savings at all for just two lines. If all you want is two lines that would be $100 TMO (versus $130 no discounts applied ATT), 10 GB per line TMO versus 15gb shared ATT, 12 month rollover tmo versus 1 month rollover ATT, and of course the music and soon to be video streaming that I mentioned above.
TMO certainly isn't for everyone. If you routinely travel outside of larger metro areas you're likely to get no data, or worse, no service. I still see LTE drop down to 4g on occasion, but I've yet to see no service except when out in the middle of nowhere (where ATT also showed no service). The good news is you can test the network on any phone they carry free of charge for a full month. If you're not happy you can simply hand back the handsets and you're on your way. You can even keep your installment plan going through T-Mobile if you wanted to keep the device and move it to another carrier (ink ear if your first month's bill will then be due, and it may only be limited to the 6s series of phones). The only caveat is it requires a credit check to get started.
